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

int64_t gcd(int64_t a, int64_t b){
    return b==0 ? a : gcd(b, a%b);
}

int main(){
    int A, B;
    cin >> A >> B;
    if(gcd(A, B) > 1){
        cout << -1 << endl;
        return 0;
    }

    bitset<20000> dp;
    dp[0] = 1;
    int ans = 0;
    for(int i=0; i<=A*B; i++){
        if(dp[i]){
            dp[i+A] = dp[i+B] = 1;
        }else{
            ans++;
        }
    }
    cout << ans << endl;
    return 0;
}