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