#include <bits/stdc++.h>
using namespace std;
int N, M;
int X;
int get(int a, int b) {
  if (a < 0 || b < 0) return 0;
  int x = a / 2, y = b / 2;
  return x * y + (a - x) * (b - y);
}
int main() {
  scanf("%d %d", &N, &M);
  scanf("%d", &X);
  N -= 2 * (X - 1);
  M -= 2 * (X - 1);
  int cur = get(N, M);
  int nxt = get(N - 2, M - 2);
  int ans = cur - nxt;
  printf("%d\n", ans);
  return 0;
}
