#include <bits/stdc++.h>
using namespace std;
int painted[12000];
void dfs(int n, int m, int x) {
  if (n <= 0 || m <= 0) return;
  painted[x] = (n * m + 1) / 2;
  dfs(n - 2, m - 2, x + 1);
}
int main() {
  int N, M, X;
  while (cin >> N >> M >> X) {
    if (X >= 10000) X = 10000;
    memset((painted), (0), sizeof(painted));
    dfs(N, M, 0);
    printf("%d\n", painted[X - 1] - painted[X]);
  }
}
