#include <bits/stdc++.h>
using namespace std;
int main() {
  int row, col, k, cnt, cell = 0, tmp, i, j;
  scanf("%d %d %d", &row, &col, &k);
  for (int x = 0; x < k; x++) {
    tmp = (2 * row + 2 * col) - 4;
    cell += tmp;
    row = row - 4;
    col = col - 4;
  }
  printf("%d\n", cell);
  return 0;
}
