#include <iostream>
#include <vector>
#include <string>
#include <algorithm>
#include <cstdio>
#include <cstring>
#include <cmath>
using namespace std;
using ll = long long;

int gcd(int n, int m) {
    if (n < m) swap(n, m);
    while (m != 0) {
        int r = n % m;
        n = m;
        m = r;
    }
    return n;
}

int main() {
    int a, b;
    cin >> a >> b;

    int c = gcd(a, b);

    if (c > 1) {
        cout << -1 << endl;
        exit(0);
    }

    if (a == 1 || b == 1) {
        cout << 0 << endl;
        exit(0);
    }

    int r = 0;
    for (int i = 1; i < a * b; i++) {
        int f = 1;
        for (int j = 0; j <= i; j += a) {
            int t = i - j;
            if (t % b == 0) {
                f = 0;
                break;
            }
        }
        r += f;
    }

    cout << r << endl;

    return 0;
}