#include <stdio.h>
#include <math.h>

int main(void) {
	int n, i=1, sum=0;
	scanf("%d",&n);
	
	do{
		sum = sum +i;
		i = i+1;
		n=n-1;
	}while(n>0);
		
	printf("sum = %d\n",sum);
	
	return 0;
}