//
//  second.cpp
//  CoderTest
//
//  Created by yuya on 2019/07/25.
//  Copyright © 2019 yuya. All rights reserved.
//

#include<bits/stdc++.h>
using namespace std;
typedef long long int ll;
typedef pair <int,int> P;
#define all(x) (x).begin(), (x).end()
#define rep(i,n) for(int (i) = 0; (i) < (int)(n); i++)
#define rep1(i,n) for(int (i) = 1; (i) <= (int)(n); i++)
#define INF 1000000
#define MOD 998244353



int main()
{
    double R;
    cin >> R;
    
    cout << setprecision(9) << 2*R*M_PI << endl;
    
    
    
    return 0;
}


