#include <bits/stdc++.h>
using namespace std;
const int INF = INT_MAX;
const int MAX = 10000001;
const int M = 1000000007;
int mod{1000000007};
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  int n, k, w;
  cin >> k >> n >> w;
  int price = k * (w * (w + 1) / 2);
  if (price <= n)
    cout << "0";
  else
    cout << price - n;
  return 0;
}
