#include <iostream>

using namespace std;

int main(){

	int data[10000+1];
	for(int i=0;i<=10000;i++){
		data[i]=0;
	}
	data[0] = 1;

	for(int i=1;i<=10000;i++){
		data[i] = data[i-1] + i;
	}

	int num;
	while(cin>>num){
		cout<<data[num]<<endl;
	}

	return 0;

}