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