#include <bits/stdc++.h>

using namespace std;

#define f first
#define s second
#define pb push_back
#define mp make_pair
#define all(v) v.begin(), v.end()
#define sz(v) (int)v.size()

#define MOO(i, a, b) for (int i=a; i<b; i++)
#define M00(i, a) for (int i=0; i<a; i++)
#define MOOd(i,a,b) for (int i = (b)-1; i >= a; i--)
#define M00d(i,a) for (int i = (a)-1; i >= 0; i--)

#define FAST ios::sync_with_stdio(0); cin.tie(0);
#define finish(x) return cout << x << '\n', 0;
#define dbg(x) cerr << ">>> " << #x << " = " << x << "\n";
#define _<< " _ " <<

typedef long long ll;
typedef long double ld;
typedef vector<int> vi;
typedef pair<int,int> pi;
typedef pair<ld,ld> pd;
typedef vector<ll> vll;
typedef pair<ll,ll> pll;

int main() { FAST
	mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
	ld pi = acos(-1);
	ld r;
	cin >> r;
	cout << r * 2.0 * pi << endl;
}
