为啥double不行，double不是比longlong大吗

#include<iostream>

using namespace std;

int main()
{
	double a1,a2,n,d;
	cin>>a1>>a2>>n;
	d=a2-a1;
	double sn=n*a1+n*(n-1)*d/2;
	cout<<sn<<endl;
	return 0;
}