#include <iostream>
#include <vector>
#include <algorithm>
#include <stdio.h>
#include <math.h>
#include <string>
#include <cctype>
#include <map>
#include <limits>
#include <iomanip>

using namespace std;

const int mod = 1000000007;
#define ARRAY_LENGTH(array) (sizeof(array) / sizeof(array[0]))
#define ll long long

int main()
{
	double r;
	cin >> r;

	double result = r * 2 * M_PI;

	cout << result << endl;
	return 0;
}
#include <bits/stdc++.h>

using namespace std;


int main(){
       string s;
       cin>>s;
       reverse  (s.begin(),s.end());
       cout<<s<<endl;

    return 0;
}
