#include <bits/stdc++.h>
using namespace std;
int main() {
  int a, b, c, d = 1;
  cin >> a >> b >> c;
  int ans = 0;
  for (int i = 0; i < c; i++) {
    ans += 2 * (a - 2 * d) + 2 * (b - 2 * d) + 4;
    d += 2;
  }
  cout << ans;
}
