#include <bits/stdc++.h>
#define F first
 #define S second
#define PB push_back
#define MP make_pair
#define ll long long int

#define PI 3.14159


using namespace std;

int main()
{
 	ios_base::sync_with_stdio(0);
	cin.tie(0);



	int r;
	
	cin >> r;
	
	float a;
	
	a = (float) 2 * PI * r;
	
	cout << a << "\n";


return 0;
}
