#include <bits/stdc++.h>
using namespace std;
int main() {
  int r, c, k, arr[105], ans;
  int first, second, sum = 0, m = 0, val = 0, leftval = 0, result = 0,
                     count = 0;
  cin >> r >> c >> k;
  val = r * c;
  while (c != 0) {
    first = c * 2;
    c = c - 2;
    r = r - 2;
    second = r * 2;
    arr[m] = (first + second);
    sum = sum + (first + second);
    m++;
    if (c == 1 || r == 1) {
      break;
    }
  }
  if (val > sum) {
    arr[m] = val - sum;
    m++;
  }
  int check = m;
  int cp = 2;
  if (k == 1) {
    cout << arr[0] << endl;
  } else {
    for (int i = 0; i < m; i = i + 2) {
      count++;
      result += arr[i];
      if (count == k) {
        break;
      }
    }
    cout << result << endl;
  }
  return 0;
}
