#include <bits/stdc++.h>
using namespace std;
int main() {
  int x, y, z, s, t, sum = 0, o;
  cin >> x >> y >> z;
  for (int i = 1; i <= z; i++) {
    if (i >= 2) {
      x -= 4;
      y -= 4;
    }
    s = x * y;
    t = (x - 2) * (y - 2);
    sum += (s - t);
  }
  cout << sum;
}
