#include <bits/stdc++.h>
using namespace std;
int a[105][105];
int main() {
  int n, k, sum = 0, i, j, ans = 0, x, y, z, x1, y1;
  cin >> x >> y >> z;
  x1 = x;
  y1 = y;
  while (z != 0) {
    sum += (x1 + y1) * 2 - 4;
    x1 -= 4;
    y1 -= 4;
    z--;
  }
  cout << sum;
}
