#include <bits/stdc++.h>
#pragma GCC optimize("Ofast")
#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,avx2,fma")
#pragma GCC optimize("unroll-loops")
using namespace std;
long long int MOD = 998244353;
double eps = 1e-12;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  long long int k, n, w, product = 0;
  cin >> k >> n >> w;
  for (int i = 1; i <= w; i++) {
    product += (i * k);
  }
  long long int res = product - n;
  if (res <= 0)
    cout << "0" << endl;
  else
    cout << res << endl;
  return 0;
}
