#include <bits/stdc++.h>
#define Phuong_ ios_base::sync_with_stdio(0); cin.tie(0);
#define pb push_back
#define mp make_pair

/// easy access -------------
using namespace std;
using ll = long long;
using ld = long double;
typedef pair<int,int> pii;
///-------------------------

int main()
{
    Phuong_; /// <3
    double n;
    cin >> n;
    cout << setprecision(10) << n*6.28318530717958623200;
    return 0;
}

