/*A - Circle Pond*/
#include <bits/stdc++.h>

using namespace std;

int main(){

	ios_base::sync_with_stdio(0);
	cin.tie(0);
	cout.tie(0);

	double radio;
	cin >> radio;

	cout << fixed << setprecision(20) << M_PI * (radio + radio) << endl;

	
	return 0;
}