#include <fstream>

using namespace std;
ifstream in("gfact.in");
ofstream out("gfact.out");

int zero(int n, int p)
{
    int nr0=0;
    while(n!=0)
    {
        nr0+=n/p;
        n/=p;
    }
    return nr0;
}

int main()
{
    const int L=30;
    int nr0,r=0,pas=1<<L,nr;

    in>>nr>>nr0;
    while(pas!=0)
    {
        if(zero(r+pas,nr)<nr0)
            r+=pas;
        pas/=2;
    }

    if(zero(r+1,nr)==nr0)
        out<<r+1;
    else
        out<<-1;
    return 0;
}