#include <bits/stdc++.h>
using namespace std;
int get(int m, int n, int x) {
  if (x > m || x > n) return 0;
  --x;
  int up = 1, down = m, left = 1, right = n;
  while (x) {
    ++up;
    --down;
    ++left;
    --right;
    --x;
  }
  if (up > down || left > right) return 0;
  int t = (down - up + 1) * (right - left + 1);
  return t / 2 + t % 2;
}
int main() {
  int m, n, x;
  while (cin >> m >> n >> x) {
    cout << get(m, n, x) - get(m, n, x + 1) << endl;
  }
  return 0;
}
