#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(0), cin.tie(NULL), cout.tie(NULL);
  int a, b, c;
  cin >> a >> b >> c;
  cout << ((a + c - 1) * (b + c - 1) - c * (c - 1)) << "\n";
  return 0;
}
