#include <bits/stdc++.h>
using namespace std;

#define ll long long
#define ull unsigned long long

int main()
{
	const double PI = 3.1415926535897932384626433832795028;
	ios_base::sync_with_stdio(false);
	cin.tie(0);
	double n;
	cin >> n;
	cout << PI*2*n;
}