#include <stdio.h>
int main(void){
    int n;
    int sum = 0;
    scanf( "%d", &n );
    if( n % 2 == 0 ){
        sum = ( n / 2 ) * ( n + 1 );
    }else{
        sum = ( ( ( n - 1 ) / 2 ) * n ) + n;
    }
    printf( "%d\n", sum );
}