#include <bits/stdc++.h>
int main() {
  int n, m, x;
  scanf("%d%d%d", &n, &m, &x);
  n -= 2 * (x - 1);
  m -= 2 * (x - 1);
  int min = n < m ? n : m;
  int max = n > m ? n : m;
  if (min <= 0)
    printf("0\n");
  else {
    max = n + m - 2 > 1 ? n + m - 2 : 1;
    printf("%d\n", max);
  }
  return 0;
}
