//Using FeatherPad
#include <bits/stdc++.h>
#define problem_solving ios::sync_with_stdio(0);cin.tie(0);
#define oo (int)1e9 + 10
#define all(x) x.begin(), x.end()
using namespace std;

void solve(int case_number) {
	double r;
	cin >> r;
	cout << fixed << setprecision(20);
	cout << r * 6.28318530717958623200 << endl;
}


int main() {
	problem_solving
	int cases = 1;
	//cin >> cases;
	for(int i=0;i<cases;i++)
		solve(i + 1);
	return 0;
}

