#include <bits/stdc++.h>
int evenglides, oddglides, width, height, glidenum, leftarea;
int main() {
  scanf("%d %d %d", &width, &height, &glidenum);
  leftarea = width * height;
  while (glidenum-- && leftarea) {
    if (width != 1 && height != 1) {
      oddglides += (leftarea) - (width -= 2) * (height -= 2);
    } else
      oddglides++;
    leftarea = width * height;
    if (width != 1 && height != 1) {
      evenglides += leftarea - ((width -= 2)) * (height -= 2);
    } else
      evenglides++;
    leftarea = width * height;
  }
  printf("%d", oddglides);
}
