#include <bits/stdc++.h>
using namespace std;
long long N, M, x;
long long rs;
int main() {
  cin >> N >> M >> x;
  if (x > N / 2 + 1 || x > M / 2 + 1)
    cout << 0;
  else {
    N -= 2 * (x - 1);
    M -= 2 * (x - 1);
    rs = N + M - 2;
    if (rs < 0)
      cout << 0;
    else if (rs == 0)
      cout << 1;
    else
      cout << rs;
  }
  return 0;
}
