#include <bits/stdc++.h>
using namespace std;
int n, m, x;
int countblack(int n, int m) {
  if (n <= 0 || m <= 0) return 0;
  if (n % 2 == 0 || m % 2 == 0)
    return (n * m) / 2;
  else
    return 1 + (n * m) / 2;
}
int count(int nn, int mm, int xx) {
  if (nn <= 0 || mm <= 0) return 0;
  if (xx == 1)
    return countblack(nn, mm) - countblack(nn - 2, mm - 2);
  else
    return count(nn - 2, mm - 2, xx - 1);
}
int main() {
  cin >> n >> m >> x;
  cout << count(n, m, x);
  return 0;
}
