#include<bits/stdc++.h>

using namespace std;

typedef long long ll;
typedef long double ld;

const ld PIE = atan2(0, -1);

void solve() {
	ld r;
	cin >> r;
	ld ans = 2.00 * PIE * r;
	cout.precision(100);
	cout << ans << "\n";
}

int main() {
	solve();
	return 0;
}