#include <bits/stdc++.h>
using namespace std;
bool dp[10005];
int main(void){
    int a, b;
    cin >> a >> b;
    if(__gcd(a, b) != 1){
        cout << -1 << endl;
        return 0;
    }
    int ans = 0;
    int ub = a * b;
    for(int i = 0; i <= b; ++i){
        for(int j = 0; j <= a; ++j){
            if(i == 0 && j == 0)    continue;
            int foo = a * i + b * j;
            if(foo > ub)    continue;
            dp[foo] = true;
        }
    }
    for(int i = 1; i <= ub; ++i)    if(!dp[i])  ++ans;
    cout << ans << endl;
}