#include <bits/stdc++.h>
using namespace std;
int n, m;
int x;
int dfs(int xx, int yy, int cc) {
  if (cc == 0) {
    if (xx == 1 || yy == 1) {
      int tt = xx * yy;
      if (tt % 2 == 0)
        return tt / 2;
      else
        return tt / 2 + 1;
    } else
      return ((xx + yy) * 2 - 4) / 2;
  }
  if (xx <= 2 || yy <= 2) {
    if (cc >= 1) return 0;
    int k = xx * yy;
    if (k % 2 == 0) return k / 2;
    return k / 2 + 1;
  }
  int g = dfs(xx - 2, yy - 2, cc - 1);
  return g;
}
int main() {
  while (cin >> n >> m >> x) {
    int t = dfs(n, m, x - 1);
    cout << t << endl;
  }
}
