#include <bits/stdc++.h>
using namespace std;
int n, m, x, yoy;
inline int read() {
  register int x = 0, f = 1;
  char ch = getchar();
  while (!isdigit(ch)) {
    if (ch == '-') f = -1;
    ch = getchar();
  }
  while (isdigit(ch)) {
    x = (x << 1) + (x << 3) + (ch ^ 48);
    ch = getchar();
  }
  return x * f;
}
int main() {
  n = read();
  m = read();
  x = read();
  n -= 2 * x - 1;
  m -= 2 * x - 1;
  if (min(n, m) < 0)
    cout << "0";
  else
    cout << max(n + m, 1);
}
