#include <bits/stdc++.h>
using namespace std;
int main() {
  int N, M, X;
  int A, B;
  scanf("%d%d%d", &N, &M, &X);
  X--;
  while (N && M && X) {
    X--;
    N = max(0, N - 2);
    M = max(0, M - 2);
  }
  A = N * M;
  if (A % 2) A++;
  A /= 2;
  B = max(N - 2, 0) * max(M - 2, 0);
  if (B % 2) B++;
  B /= 2;
  printf("%d\n", A - B);
  return 0;
}
