#include <bits/stdc++.h>
using namespace std;
const double inf = 0.00001;
int dfs(int r, int c, int z) {
  if (z == 0) {
    if (r == 1 || c == 1) {
      int k = r * c;
      if (k % 2 == 0) return k / 2;
      return k / 2 + 1;
    }
    return (2 * (r + c) - 4) / 2;
  }
  if (r <= 2 || c <= 2) {
    if (z >= 1) return 0;
    int k = r * c;
    if (k % 2 == 0) return k / 2;
    return k / 2 + 1;
  }
  return dfs(r - 2, c - 2, z - 1);
}
int main() {
  int a, b, c;
  cin >> a >> b >> c;
  cout << dfs(a, b, c - 1) << endl;
}
