#include <bits/stdc++.h>

using namespace std;

#define MX 35
#define MOD 1'000'000'007
#define F first
#define S second
#define br '\n'

typedef long long ll;
typedef vector<int> vi;
typedef pair<int,int> pii;

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

	double r;
	cin >> r;
	cout << setprecision(10) << acos(-1) * r * 2 << endl;

	return 0;
}
