#include <bits/stdc++.h>
using namespace std;
int main() {
  int k, n, w;
  int rate;
  int cost = 0;
  int loan;
  cin >> k >> n >> w;
  for (int i = 1; i <= w; i++) {
    rate = i * k;
    cost += rate;
  }
  loan = cost - n;
  if (loan < 0) loan = 0;
  cout << loan << endl;
  return 0;
}
