#include <bits/stdc++.h>
using namespace std;
int n, m, k;
int main() {
  scanf("%d%d%d", &n, &m, &k);
  --k;
  n -= 2 * k;
  m -= 2 * k;
  if (n <= 0 || m <= 0) {
    puts("0");
    return 0;
  }
  if (n == 1 && m == 1)
    puts("1");
  else
    cout << (n + m - 2) << endl;
}
