#include <iostream>
#include <cstdio>
#include <stdlib.h>
#include <ctype.h>
#include <cstring>
#include <math.h>


int main(){
    double R;
    double A =3.141592653589793;
    scanf("%lf",&R);
    printf("%.15f",A*2*R);


    return 0;
}