#include <bits/stdc++.h>
using namespace std;
#define rep(i,a,b) for(auto i = (a); i < (b); i++)
#define iter(it,c) for(auto it = (c).begin(); it != (c).end(); ++it)
typedef long long ll;
typedef vector<int> vi;
typedef pair<int,int> ii;
typedef vector<ii> vii;
const int INF = ~(1<<31);
#define fs first
#define sc second
#define pb push_back

const double EPS = 1e-9;
const double pi = acos(-1);
typedef unsigned long long ull;
typedef vector<vi> vvi;
typedef vector<vii> vvii;
template <class T> T smod(T a, T b) {
    return (a % b + b) % b;
}

int main() {
    cin.sync_with_stdio(false);
    double r;
    cin >> r;
    cout << setprecision(15) << fixed << 2*r*pi << endl;
    return 0;
}
