#include <stdio.h>

int main() {
    int day = 0;
    int sum = 0;
    int coin = 1;
    int count = 0;
    scanf("%d", &day);
    for (int i = 1; i <= day;i++ ) {//用来计算天数
        sum += coin;
        count++;
        if (count == coin)
        {
            coin++;
            count = 0;
        }
    }
    printf("%d", sum);
    return 0;
}