/+ dub.sdl:
    name "A"
    dependency "dcomp" version=">=0.3.2"
    lflags "-stack_size" "100000000"
+/

import std.algorithm, std.conv, std.range, std.stdio;

// import dcomp.scanner;
// import dcomp.algorithm;

int main(string[] argv) {
    auto sc = new Scanner();
    int n;
    string s, t;
    sc.read(n, s, t);

    //first check
    if (s == t) {
        writeln(0);
        return 0;
    }
    if (s[0] != t[0]) {
        writeln(-1);
        return 0;
    }

    int[][26] pos;
    foreach (i; 1..n) {
        int id = s[i] - 'a';
        pos[id] ~= i;
    }
    int ma = 0;
    int[2] p = [n, 0];
    foreach_reverse(i; 1..n) {
        if (t[i-1] == t[i]) continue;
        int id = t[i] - 'a';
        int r = min(p[0]-1, i);
        //check t[i], search id, [1, r]
        int lt = binSearch!(i => pos[id][i] > r)(-1, pos[id].length.to!int)-1;
        if (lt == -1) {
            writeln(-1);
            return 0;
        }
        int nw = pos[id][lt];
        if (i < p[0]) {
            p = [nw, 0];
        } else {
            p = [nw, p[1]+1];
        }
        if (nw != i) {
            ma = max(ma, p[1]+1);
        }
    }
    writeln(ma+1);
    return 0;
}
/* IMPORT /Users/yosupo/Program/dcomp/source/dcomp/scanner.d */
// module dcomp.scanner;

class Scanner {
    import std.stdio : File, stdin;
    import std.conv : to;
    import std.range : front, popFront, array, ElementType;
    import std.array : split;
    import std.traits : isSomeChar, isStaticArray, isArray; 
    import std.algorithm : map;
    File f;
    this(File f = stdin) {
        this.f = f;
    }
    string[] buf;
    private bool succ() {
        while (!buf.length) {
            if (f.eof) return false;
            buf = f.readln.split;
        }
        return true;
    }
    private bool readSingle(T)(ref T x) {
        if (!succ()) return false;
        static if (isArray!T) {
            alias E = ElementType!T;
            static if (isSomeChar!E) {
                //string or char[10] etc
                x = buf.front;
                buf.popFront;
            } else {
                static if (isStaticArray!T) {
                    //static
                    assert(buf.length == T.length);
                }
                x = buf.map!(to!E).array;
                buf.length = 0;                
            }
        } else {
            x = buf.front.to!T;
            buf.popFront;            
        }
        return true;
    }
    int read(T, Args...)(ref T x, auto ref Args args) {
        if (!readSingle(x)) return 0;
        static if (args.length == 0) {
            return 1;
        } else {
            return 1 + read(args);
        }
    }
}

unittest {
    import std.path : buildPath;
    import std.file : tempDir;
    import std.algorithm : equal;
    import std.stdio : File;
    string fileName = buildPath(tempDir, "kyuridenanmaida.txt");
    auto fout = File(fileName, "w");
    fout.writeln("1 2 3");
    fout.writeln("ab cde");
    fout.writeln("1.0 1.0 2.0");
    fout.close;
    Scanner sc = new Scanner(File(fileName, "r"));
    int a;
    int[2] b;
    char[2] c;
    string d;
    double e;
    double[] f;
    sc.read(a, b, c, d, e, f);
    assert(a == 1);
    assert(equal(b[], [2, 3]));
    assert(equal(c[], "ab"));
    assert(equal(d, "cde"));
    assert(e == 1.0);
    assert(equal(f, [1.0, 2.0]));
}
/* IMPORT /Users/yosupo/Program/dcomp/source/dcomp/algorithm.d */
// module dcomp.algorithm;

//[0,0,0,...,1,1,1]で、初めて1となる場所を探す。pred(l) == 0, pred(r) == 1と仮定
T binSearch(alias pred, T)(T l, T r) {
    while (r-l > 1) {
        T md = (l+r)/2;
        if (!pred(md)) l = md;
        else r = md;
    }
    return r;
}

import std.range.primitives;

Rotator!Range rotator(Range)(Range r)
if (isForwardRange!Range && hasLength!Range) {
    return typeof(return)(r);
}

struct Rotator(Range)
if (isForwardRange!Range && hasLength!Range) {
    size_t cnt;
    Range start, now;
    this(Range r) {
        cnt = 0;
        start = r.save;
        now = r.save;
    }
    this(this) {
        start = start.save;
        now = now.save;
    }
    @property bool empty() {
        return now.empty;
    }
    @property auto front() {
        assert(!now.empty);
        import std.range : take, chain;
        return chain(now, start.take(cnt));
    }
    @property Rotator!Range save() {
        return this;
    }
    void popFront() {
        cnt++;
        now.popFront;
    }
}


E minimum(alias pred = "a < b", Range, E = ElementType!Range)(Range range, E seed)
if (isInputRange!Range && !isInfinite!Range && !is(CommonType!(ElementType!Range, E) == void)) {
    import std.functional;
    while (!range.empty) {
        if (binaryFun!pred(range.front, seed)) {
            seed = range.front;
        }
        range.popFront;
    }
    return seed;
}

ElementType!Range minimum(alias pred = "a < b", Range)(Range range)
if (isInputRange!Range && !isInfinite!Range) {
    assert(!range.empty, "range must not empty");
    auto e = range.front; range.popFront;
    return minimum!pred(range, e);
}

unittest {
    assert(minimum([2, 1, 3]) == 1);
    assert(minimum!"a > b"([2, 1, 3]) == 3);
    assert(minimum([2, 1, 3], -1) == -1);
    assert(minimum!"a > b"([2, 1, 3], 100) == 100);
}

bool[ElementType!Range] toMap(Range)(Range r) {
    import std.algorithm : each;
    bool[ElementType!Range] res;
    r.each!(a => res[a] = true);
    return res;
}
