#include <bits/stdc++.h>
int main() {
  int w, h, k, row, col, area;
  while (scanf("%d%d%d", &w, &h, &k) == 3) {
    area = 0;
    for (int i = 1; i <= k; i++) {
      row = w - 4 * (i - 1);
      col = h - 4 * (i - 1);
      area += (row + col - 2) * 2;
    }
    printf("%d\n", area);
  }
  return 0;
}
