#include <stdio.h>

#define DEF_INPUT_MIN (1)
#define DEF_INPUT_MAX (100)

int main(void) {
    int itr = 0;
    int input = 0;
    int output = 0;

    while(1) {
        scanf("%d", &input);
        if(input < DEF_INPUT_MIN || input > DEF_INPUT_MAX) {
            continue;
        }
        
        for (itr = 0; itr <= input ; itr++) {
            output += itr;
        }
        printf("%d\n", output);
        break;
    }
    return 0;
}