等差数列前n项和

#include<iostream.h>
void main()
{
int s,i,n,sum;
cout<<"输入初始值s："<<"\t";
cin>>s;
cout<<"输入增量i:"<<"\t";
cin>>i;
cout<<"输入要求的前n项:"<<"\t";
cin>>n;
sum=(s+s+(n-1)*i)*n/2;
cout<<"该等差数列前"<<n<<"项和为："<<sum<<endl;

}