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

int main() {
    int a, b;
    cin >> a >> b;
    if (gcd(a, b) != 1) {
        cout << -1 << endl;
        return 0;
    }
    vector<bool> good(a * b);
    for (int i = 0; i < a; i++) {
        for (int j = 0; j < b; j++) {
            if (b * i + a * j >= a * b) continue;
            good.at(b * i + a * j) = true;
        }
    }
    int ans = 0;
    for (int i = 0; i < a * b; i++) ans += !good.at(i);
    cout << ans << endl;
}