#include<bits/stdc++.h>
using namespace std;
#define fast ios_base::sync_with_stdio(false);cin.tie(NULL);cout.tie(NULL);
#define endl "\n"
#define watch(x); cout<<#x<<"="<<x<<endl;
#define watch2(x,y); cout<<#x<<"="<<x<<" "<<#y<<"="<<y<<endl;
#define int long long

int32_t main() 
{ 
    fast
    double r;
    double pi=3.14159265358979323846264;
    cin>>r;
   	r=2*pi*r;
   	cout<<setprecision(10)<<r;


    return 0; 
} 
