#include<iostream>
#include<cstdio>
#include<string>

using namespace std;

int main()
{	
	int ans[10001]={0};
	int n;
	ans[1]=2;
	for(int i=2;i<=10001;i++){
		ans[i]=ans[i-1]+i;
	}
	while(cin>>n){
		cout<<ans[n]<<endl;
	}
	return 0;
}