#include <bits/stdc++.h>
using namespace std;
int main() {
  int a, b, c, tiles = 0;
  cin >> a >> b >> c;
  for (int i = 0; i < a - 1; i++, b++) tiles += 2 * b;
  tiles += (c - a + 1) * b;
  cout << tiles << endl;
  return 0;
}
