A. Soldier and Bananas Solution

#include<bits/stdc++.h>

using namespace std;

int main()

{

    long long int w,k,n,sum,need;

    cin>>k>>n>>w;

    sum=(w*(w+1))/2;

    sum=sum*k;

    need=sum-n;

    if(need>=0)

        cout<<need<<endl;

    else

        cout<<"0"<<endl;

    return 0;

}