#include<bits/stdc++.h>

using namespace std;

typedef long long int ll;

void solve()
{
	int r;
	cin >> r;
	cout << fixed << setprecision(3) << 2*3.141592*r;
}

int main()
{
	int t = 1;
	//cin >> t;
	for(int i = 0; i < t; i++)
	{
		//cout << "Case #" << i + 1 << ": ";
		solve();
	}
	return 0;
}

