A soldier wants to buy *w* bananas in the shop. He has to pay *k* dollars for the first banana, 2*k* dollars for the second one and so on (in other words, he has to pay *i*·*k* dollars for the *i*-th banana). 

He has *n* dollars. How many dollars does he have to borrow from his friend soldier to buy *w* bananas?
The first line contains three positive integers *k*, *n*, *w* (1  ≤  *k*, *w*  ≤  1000, 0 ≤ *n* ≤ 10<sup class="upper-index">9</sup>), the cost of the first banana, initial number of dollars the soldier has and number of bananas he wants.
/**
 *  author: Ratem
 *  created: 12.10.2023 12:58:39
**/

#include <bits/stdc++.h>

using namespace std;
typedef long long ll;

#ifdef LOCAL
#include "algo/debug.h"
#else
#define debug(...) 42
#endif

int main()
{
    ll k, n, w;
    cin >> k >> n >> w;

    for (int i = 1; i <= w; i++)
    	n -= i * k;

    printf("%d\n", abs(n));
    return 0;
}
