#include<bits/stdc++.h>
using namespace std;

typedef long long int ll;

#define fast ios_base::sync_with_stdio(false);cin.tie(NULL);cout.tie(NULL);
#define endl "\n"

int main()
{
     fast;
	float r,res;
	float pi = 3.141592653
    cin>>r;
  	cout<<2*pi*r<<endl;
     return 0;
}