#include<bits/stdc++.h>

#define int long long
#define ar array 
#define mp make_pair
#define fi first
#define se second
#define pb push_back
#define all(x) (x).begin(), (x).end()
#define rall(x) (x).rbegin(), (x).rend()
#define rep(i,a,b) for(int i = a; i < b; i++)
 
using namespace std;

typedef long long ll; 
typedef vector<int> vi;
typedef pair<int,int> pii;
 
 
const  int mxN = 1e5+5;
const  long long INF= 1e18;
const long long mod= 1e9+7;
 	
signed main(){
	double r;
	cin >> r;
	r = r * 2;
	r = r * 3.14159265359;
	cout << r;
}
	
	
	
	
	
	
	
	
	
	
	
	
	
	
	
	
	
	
	
	

