#include <iostream>
using namespace std;

int coprime(int a, int b){
    if(a > b){
        int tmp = a;
        a = b;
        b = tmp;
    }
    for(int i = 2; i <= a; i++){
        if(a % i == 0 && b % i == 0) return false;
    }
    return true;
}

int main(void){
    int a, b;

    cin >> a >> b;
    if(coprime(a, b) == false){
        cout << -1 << endl;
        return 0;
    }

    cout << (a - 1) * (b - 1) / 2 << endl;

    return 0;
}