/*
 ID: swagchicken1
 PROG:
 LANG: C++11
 */

#include <iostream>
#include <tuple>
#include <cmath>
#include <string>
#include <cstring>
#include <vector>
#include <deque>
#include <queue>
#include <stack>
#include <map>
#include <unordered_map>
#include <set>
#include <unordered_set>
#include <algorithm>
#include <vector>
#include <fstream>
#include <iomanip>
#include <ctime>
#include <cctype>
#include <climits>
#include <chrono>

using namespace std;

typedef long long ll;
typedef vector<int> vi;
typedef vector< vector <int> > vvi;
typedef pair<int, int> pii;
typedef pair < pair < int, int >, int > piii;
typedef pair < pair <int, int > , pair <int, int> > piiii;

#define FOR(i,a,b) for(int i = a; i < b; i ++)
#define RFOR(i,a,b) for(int i = a-1; i >= b; i --)

//#define endl '\n';

#define mp make_pair
#define pb push_back
#define f first
#define s second

const int MOD = 998244353;

long double PI = 4*atan(1);
long double eps = 1e-12;



int main() {
    //auto start = chrono::high_resolution_clock::now();
    ios_base::sync_with_stdio(0);cin.tie(0);
    //ofstream cout("output.txt");
    //ifstream cin("input.txt");
    int n; cin >> n; cout << 2 * PI * n << endl;
    
    
    
    //auto stop = chrono::high_resolution_clock::now();
    //auto duration = chrono::duration_cast<chrono::microseconds>(stop - start);
    //cout << duration.count() << endl;
    //cin.close();
    //cout.close();
}
