///بِسْمِ الّٰلهِ الرَّحْمٰنِ الرَحِيْمِ
#include<bits/stdc++.h>
using namespace std;
#define ll long long
#define pi acos(-1.0)
int main()
{
     ios_base::sync_with_stdio(false);cin.tie(NULL);
     ll int i,n,t,p=INT_MAX,j=0,x,a=0,b,l=0,r=0,y,k,c,sum=0,m,d;
     cin>>r;
     double sd=2*pi*r;
     printf("%.5f",sd);
     return EXIT_SUCCESS;
}

