#include <bits/stdc++.h>
using namespace std;
int main() {
  int i, j, n;
  cin >> i >> j >> n;
  int s = 0;
  int c = 0;
  int e = 0;
  int d = 0;
  int sum = 0;
  while (s < n) {
    if (s == 0) {
      c = i * 2 + (j - 2) * 2;
      sum += c;
    } else {
      d = i - (4 * s);
      e = j - (4 * s);
      e -= 2;
      sum += (2 * d + 2 * e);
    }
    s++;
  }
  cout << sum;
  return 0;
}
