#include <bits/stdc++.h>
#define rep(i, n) for(int i = 0; i < (n); i++)
using namespace std;

signed main(void)
{
	double r;
	cin >> r;
	cout << fixed << setprecision(20) <<(double) r * 2 * 3.141592653589793 << endl;
	return 0;
}
