#include <bits/stdc++.h>
using namespace std;
inline int max2(int a, int b) { return ((a > b) ? a : b); }
inline int min2(int a, int b) { return a < b ? a : b; }
inline int max3(int a, int b, int c) { return max2(a, max2(b, c)); }
long long int gcd(long long int a, long long int b) {
  while (b) {
    long long int temp = a;
    a = b;
    b = temp % b;
  }
  return a;
}
int main() {
  int n, w, k;
  scanf("%d", &k);
  scanf("%d", &n);
  scanf("%d", &w);
  int total = w * (w + 1);
  total = total / 2;
  total = total * k;
  int diff = total - n;
  if (diff > 0)
    printf("%d\n", diff);
  else
    printf("%d\n", 0);
  return 0;
}
