#include <iostream>
#include <vector>
#include <algorithm>

#define INPUT_FROM_FILE (0) 
#if INPUT_FROM_FILE
#include <fstream>
#endif

int main(){
	int n;

#if INPUT_FROM_FILE
	std::ifstream ifs("test.txt");
	//FILE *fp = fopen("test.txt", "r");
#endif


#if INPUT_FROM_FILE
	while(ifs >> n){
#else
	while(std::cin >> n){
#endif
		std::cout << 1 + n * (n + 1) / 2 << std::endl;
	}

#if INPUT_FROM_FILE
	//fclose(fp);
#endif
	return 0;
}
