typedef long long ll;
#include <bits/stdc++.h>
using namespace std;

int main() {
    ll a,b;
    std::cin >> a>>b;
    if(gcd(a,b)!=1){
        std::cout << -1 << std::endl;
        return 0;
    }
    ll cnt = 0;
    for (int i = 1; i <= 10000; i++) {
        bool ok = false;
        for (int j = 1; j <= i; j++) {
            // if(i-a*j<0)continue;
            
            if((i-a*j)%b==0&&(i-a*j)>=0){
                ok = true;
                break;
            }
            if((i-b*j)%a==0&&(i-b*j)>=0){
                ok = true;
                break;
            }
        }
        if(!ok)cnt++;
    }
    std::cout << cnt << std::endl;
}