#include <bits/stdc++.h>
using namespace std;
int main() {
  long long n, i, s = 0, h, w, k, g = 0;
  cin >> w >> h >> k;
  for (i = 1; i <= k; i++) {
    s = (s + (w * 2 - g - g - g - g) + (h * 2 - g - g - g - g)) - 4;
    g = g + 2;
  }
  cout << s;
  return 0;
}
