Codeforce 546 A. Soldier and Bananas

#include <iostream>
#include <cstdio>
#include <algorithm>

using namespace std;

int main()
{
int k,n,w;
scanf("%d%d%d",&k,&w,&n);
long long sum=0,s=1;
sum=k*(1+n)*n/2;
s=sum-w;
if(s<=0)
printf("0\n");
else
printf("%lld\n",s);

return 0;
}