#include <bits/stdc++.h>
using namespace std;
int main() {
  int a, b, c, d;
  while (~scanf("%d%d%d", &a, &b, &c)) {
    int ans = 0;
    for (int i = 1; i <= min(b, c); i++) {
      ans += a;
      a++;
    }
    int t;
    if (b > min(b, c))
      t = b - min(b, c);
    else
      t = c - min(b, c);
    if (t == 0)
      printf("%d\n", ans * 2 - (a - 1));
    else
      printf("%d\n", ans * 2 + (t - 1) * (a - 1));
  }
}
