#include <bits/stdc++.h>
using namespace std;
const int N = 5e5 + 1;
const int L = 3e4 + 1;
long long n, m, l = 1, r, q, b[N], a[101][101], ans;
string s;
int main() {
  ios_base::sync_with_stdio(false);
  cin >> n >> m >> q;
  for (int z = 1; z <= q; ++z) {
    for (int i = l; i <= n + 1 - l; ++i) {
      for (int j = l; j <= m + 1 - l; ++j) {
        if (i == l) a[i][j] = 1;
        if (j == l) a[i][j] = 1;
        if (i == n + 1 - l) a[i][j] = 1;
        if (j == m + 1 - l) a[i][j] = 1;
      }
    }
    l += 2;
  }
  for (int i = 1; i <= n; ++i) {
    for (int j = 1; j <= m; ++j) {
      if (a[i][j] == 1) ans++;
    }
  }
  cout << ans;
}
