#include <bits/stdc++.h>
int main(void) {
  int k, n, w, fall = 0, ans = 0;
  scanf("%d %d %d", &k, &n, &w);
  for (int i = 1; i <= w; i++) {
    fall += k * i;
  }
  ans = fall - n;
  if (ans >= 0) {
    printf("%d", ans);
  } else {
    printf("%d", 0);
  }
  return 0;
}
