import std.stdio, std.string, std.conv;
import std.range, std.algorithm, std.array, std.typecons, std.container;
import std.math, std.numeric, core.bitop;



void main() {
    long n, m;
    scan(n, m);
    auto s = readln.chomp;
    auto t = readln.chomp;

    auto l = lcm(n, m);
    auto sg = l / m;
    auto tg = l / n;

    foreach (i ; 0 .. n / sg) {
        if (s[i*sg] != t[i*tg]) {
            writeln(-1);
            return;
        }
    }

    writeln(l);
}

long lcm(long a, long b) {
    return a / gcd(a, b) * b;
}

























void scan(T...)(ref T args) {
    import std.stdio : readln;
    import std.algorithm : splitter;
    import std.conv : to;
    import std.range.primitives;

    auto line = readln().splitter();
    foreach (ref arg; args) {
        arg = line.front.to!(typeof(arg));
        line.popFront();
    }
    assert(line.empty);
}


void fillAll(R, T)(ref R arr, T value) {
    static if (is(typeof(arr[] = value))) {
        arr[] = value;
    }
    else {
        foreach (ref e; arr) {
            fillAll(e, value);
        }
    }
}