#include <cstdio>
#include <cmath>
#include <limits>
#include <iostream>
#include <algorithm>
#include <vector>
#include <utility>
#include <cstring>
#include <set>
#include <map>
#include <queue>
#include <stack>
#define rep(i, s, g) for (i = s; i < g; i++)
using namespace std;
using ll = long long;
const int inf = 1000000000; //10^9
const  double PI = 3.1415926535897932;

int main(){
    double R,ans;
    cin >> R ;
    ans = 2*R*PI;

    cout << ans << endl;
}