#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, m, c;
  cin >> n >> m >> c;
  c--;
  n -= c * 2;
  m -= c * 2;
  if (n <= 0 || m <= 0) {
    cout << 0 << endl;
    return 0;
  }
  if (n == 1 || m == 1) {
    int ans = (n + m) / 2;
    cout << ans << endl;
    return 0;
  }
  int mark;
  int ans = 0;
  if (n & 1) {
    mark = (m + 1) / 2;
    ans += 2 * mark;
  } else
    ans += m;
  if (m & 1) {
    mark = (n + 1) / 2;
    ans += 2 * mark;
  } else
    ans += n;
  if ((n & 1) && (m & 1))
    ans -= 4;
  else
    ans -= 2;
  cout << ans << endl;
  ;
  return 0;
}
