#include <algorithm>
#include <bits/stdc++.h>
using namespace std;
#define rep(i, n) for (int i = 0; i < (int)(n); i++)
#define all(x) (x).begin(), (x).end()
#define ll long long
#define INF 1000000000000000000

long long GCD(long long a, long long b) {
    if (b == 0)
        return a;
    else
        return GCD(b, a % b);
}

int main() {
    cin.tie(0);
    ios::sync_with_stdio(false);

    int A, B;
    cin >> A >> B;
    if (GCD(A, B) != 1) {
        cout << -1 << endl;
    } else {
        int cnt = 0;
        for (int i = 0; i <= max(A, B); i++) {
            for (int j = 0; j <= max(A, B); j++) {
                if (A * i + B * j < A * B) {
                    cnt++;
                }
            }
        }

        cout << A * B - cnt << endl;
    }
}