#include <bits/stdc++.h>
using namespace std;
#define itn int
#define nibu(K, x) binary_search(K.begin(), K.end(), x)
#define rep(i, n) for (int i = 0; i < n; i++)
#define all(x) x.begin(), x.end()
#define sz(x) x.size()
#define pb push_back
#define mp make_pair
#define F first
#define S second
#define out(x) cout << x << endl
#define put(x) out(x)
#define in(x) cin >> x
#define maxx(a, b, c) max(a, max(b, c))
#define minn(a, b, c) min(a, min(b, c))
using ll = long long;
using vl = vector<ll>;
using vs = vector<string>;
const double pi = 3.14159265358979;
const ll mod = 1000000007;

int main()
{
    double N;
    in(N);
    cout << fixed << setprecision(10) << N * 2 * pi << endl;
}
