#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef long double ld;

template <typename T>
using lim = numeric_limits<T>;

const double PI = acos(-1);
int main(){
	ios_base::sync_with_stdio(false);
	cin.tie(0);

	double r; cin >> r;
	cout << r*2*PI << "\n";

}