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