#include<bits/stdc++.h>

#define pb push_back
using namespace std;
 
int main()
{
 
 	#ifndef ONLINE_JUDGE
  

    ios_base::sync_with_stdio(false);
    cin.tie(0);cout.tie(0);

 	int r;
 	cin>>r;
 	cout<<(float)2*M_PI*r;
 	


	return 0;
}