#include <bits/stdc++.h>
#define MAX (ll)(1e5 + 7)
#define INF (ll)(3e18)
#define MOD (ll)(998244353)

using namespace std;
using ll = long long;

int main () {
	double pi = 3.14159;
	double r;
	cin >> r;
	cout << setprecision (5) << (2.0 * pi * r) << "\n";
}
