#define _CRT_SECURE_NO_WARNINGS
#include<iostream>
#include<sstream>
#include<string>
#include<algorithm>
#include<math.h>
#include<iomanip>
#include<vector>
#include<queue>
#include<deque>
#include<list>
#include<set>
#include<map>
#include<bitset>
#include<iterator>
#include <stack>
#define ll long long
typedef long double ld;
#define all(v) ((v).begin()),((v).end())
#define vec2(n,m) vector<vector<ll>>vv(n + 10, vector<ll>(m + 10, 0));
#define p_q1   priority_queue<int,vector<int>,greater<int>>pq1;
#define format(n) fixed<<setprecision(n)
#define ll_min LLONG_MIN;
#define ll_max LLONG_MIN;
#define endl "\n"
#define F first 
#define S second
#define mkpr(x,y)  make_pair(x,y)
using namespace std;
void weza()
{
    std::ios_base::sync_with_stdio(0);
    cin.tie(NULL);
    cout.tie(NULL);
}
int main()
{
    weza();
    double n;
    cin >> n;
    cout << format(2);
    cout << 2 * n * 22/7<< endl;
}