#include <bits/stdc++.h>
using namespace std;
const int N = -1;
const long long INF = 1e9 + 19;
int main() {
  int n, m, x;
  scanf("%d%d%d", &n, &m, &x);
  x--;
  auto f = [&](int y) {
    int nn = n - y * 2;
    int mm = m - y * 2;
    if (nn <= 0 || mm <= 0) {
      return 0;
    }
    return (nn * mm + 1) / 2;
  };
  cout << f(x) - f(x + 1) << endl;
  return 0;
}
