#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, m, x;
  scanf("%d%d%d", &n, &m, &x);
  --x;
  if (x > (min(n, m) - 1) / 2) {
    puts("0");
    exit(0);
  }
  n -= x * 2;
  m -= x * 2;
  int ans = 0;
  ans += m + 1 >> 1;
  if (n > 1) {
    ans += m + (n & 1) >> 1;
    ans += (n - 2) >> 1;
    if (m > 1) ans += (n - 2 + (~m & 1)) >> 1;
  }
  printf("%d\n", ans);
}
