#include <bits/stdc++.h>
using namespace std;
int main() {
  int k, w, n;
  int j;
  cin >> k >> n >> w;
  for (int i = 1; i <= w; i++) {
    j = i * k;
  }
  int net = w * (j + k) / 2;
  int result = net - n;
  if (result > 0)
    cout << result;
  else
    cout << 0;
}
