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