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