///بِسْمِ الّٰلهِ الرَّحْمٰنِ الرَحِيْمِ7
#include<bits/stdc++.h>
#define ll long long
#define pi acos(-1.0)
int main()
{

    float r;
     scanf("%f",&r);
     float sd=2*pi*r;
     printf("%.5f",sd);
     return EXIT_SUCCESS;
}

