import core.bitop, std.algorithm, std.ascii, std.bigint, std.conv, std.math,
    std.functional, std.numeric, std.range, std.stdio, std.string, std.random,
    std.typecons, std.container, std.format;

// dfmt off
T lread(T = long)(){return readln.chomp.to!T();}
T[] lreads(T = long)(long n){return generate(()=>readln.chomp.to!T()).take(n).array();}
T[] aryread(T = long)(){return readln.split.to!(T[])();}
void scan(TList...)(ref TList Args){auto line = readln.split();
foreach (i, T; TList){T val = line[i].to!(T);Args[i] = val;}}
alias sread = () => readln.chomp();enum MOD = 10 ^^ 9 + 7;
alias PQueue(T, alias less = "a<b") = BinaryHeap!(Array!T, less);
// dfmt on

void main()
{
    long H, N;
    scan(H, N);
    auto A = new long[](N);
    auto B = new long[](N);
    foreach (i; 0 .. N)
        scan(A[i], B[i]);
    alias T = Tuple!(long, long);
    PQueue!(T, "b<a") PQ;
    PQ.insert(T(0, H));
    auto ans = new long[](H + 1);
    ans[] = -1;
    while (!PQ.empty)
    {
        auto t = PQ.front;
        PQ.removeFront();
        if (ans[t[1]] != -1)
        {
            continue;
        }
        ans[t[1]] = t[0];
        foreach (i; 0 .. N)
        {
            PQ.insert(T(t[0] + B[i], max(0, t[1] - A[i])));
        }
    }
    writeln(ans[0]);
}
