    
/*
CODED BY               
**************
DARSHAN JAIN
darshanjain31
**************
*/
#include<bits/stdc++.h>
using namespace std;
#define ll long long int
#define pb push_back
#define mp make_pair
#define fast ios_base::sync_with_stdio(false); cin.tie(NULL);
#define endl '\n'
#define MOD 1000000007
using namespace std;

int main()
{
	fast;
	ll t;
	cin>>t;
	double mid=(double)2*3.14159*t;
	cout << std::setprecision(5) <<mid<<endl;  
}





