#include <bits/stdc++.h>
int n, m;
int main() {
  int i, j, T;
  long long k, total, w;
  while (scanf("%I64d%I64d%I64d", &k, &total, &w) != EOF) {
    long long ans = (1 + w) * w / 2;
    ans *= k;
    if (ans - total >= 0)
      printf("%I64d\n", ans - total);
    else
      printf("0\n");
  }
  return 0;
}
