#include <bits/stdc++.h>

using namespace std;

#define intt long long
#define FAST ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0);

const int N = 100005;
const double PI = 3.14159265359;

int n;
int a[N];

int main() {

	FAST;

	int r;
	cin >> r;
	double ans = 2 * PI * r;
	cout << setprecision(9) << fixed << ans << '\n';
	return 0;
}