#include "stdio.h"
void fun(int n,int k)
{ int j;double sum=0;
for(j=1;j<=n;j++)
{
sum=sum+k*j;
}
printf("\n%f\n",sum);
}
main()
{int n,k;
printf("Please enter n,k:");
scanf("%d%d",&n,&k);
fun(n,k);
}