#include <bits/stdc++.h>
using namespace std;
int gcd(int a, int b){
  if (b == 0){
    return a;
  } else {
    return gcd(b, a % b);
  }
}
int main(){
  int A, B;
  cin >> A >> B;
  if (gcd(A, B) > 1){
    cout << -1 << endl;
  } else {
    cout << (A - 1) * (B - 1) / 2 << endl;
  }
}