#include <bits/stdc++.h>
using namespace std;
int main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  int k, n, w;
  cin >> k >> n >> w;
  int jml = 0;
  for (int i = 1; i <= w; ++i) jml += (i * k);
  if (jml > n)
    cout << jml - n << "\n";
  else
    cout << 0 << "\n";
  return 0;
}
