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