#include <bits/stdc++.h>
using namespace std;
int main() {
  int a, b, c;
  cin >> a >> b >> c;
  int chet = 0;
  for (int u = 0; u < c; u++) {
    chet += (a + b - 2) * 2 - u * 16;
  }
  cout << chet;
  return 0;
}
