#include <bits/stdc++.h>
using namespace std;
int main() {
  int costB, initDoll, numBan;
  int borrow = 0, tmp = 0;
  cin >> costB >> initDoll >> numBan;
  while (numBan != 0) {
    if (initDoll >= (costB * numBan)) {
      initDoll -= (costB * numBan);
    } else {
      while (initDoll < (costB * numBan)) {
        initDoll += 1;
        borrow += 1;
      }
      initDoll -= (costB * numBan);
    }
    numBan--;
  }
  cout << borrow;
}
