#include<stdio.h>
#include<math.h>
int main()
{
    int R;
    double pi = 3.1416,circumference;

    scanf("%d",&R);

    if(R>0 && R<101)
    {
        circumference = (2*pi*R);

    printf("%.2lf\n",circumference);
    }

    else{
        printf("this number not allow");
    }


}
