//
//  main.c
//  B - Round Pond
//
//  Created by MD.SAKIBUL ISLAM  on 22/4/20.
//  Copyright © 2020 MD.SAKIBUL ISLAM . All rights reserved.
//

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

int main()
{
    double a;
    int r;
    scanf("%d",&r);
    a=2*r*M_PI;
    printf("%lf\n",a);
    return 0;
}
