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