#include <iostream>
using namespace std;
int main()
{
 double U = 3.14;
 int R;
cin >> R  >> U; 
 cout <<  R * 2 * U <<"\n";
}
#include <bits/stdc++.h>

using namespace std;
typedef long long ll;

int main () {
    ios_base::sync_with_stdio(false);
    cin.tie(0);
	int r; cin >> r;
  	cout << 2 * r * acos(-1) << endl;
  	return 0;
}
