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