#include <bits/stdc++.h>
using namespace std;
const int MaxN = 10004, NA = -1, MaxC = 0x3F3F3F3F;
int m, n, x;
int f(int n, int m, int x) {
  n -= x + x;
  m -= x + x;
  if (n <= 0 || m <= 0) return 0;
  return (n * m + 1) >> 1;
}
int main(void) {
  while (scanf(" %d %d %d", &n, &m, &x) != EOF) {
    printf("%d\n", f(n, m, x - 1) - f(n, m, x));
  }
  return 0;
}
