
// Problem : A - Circle Pond
// Contest : AtCoder - AtCoder Beginner Contest 163
// URL : https://atcoder.jp/contests/abc163/tasks/abc163_a
// Memory Limit : 1024 MB
// Time Limit : 2000 ms
// Powered by CP Editor (https://github.com/cpeditor/cpeditor)

#include <bits/stdc++.h>
using namespace std;

int32_t main()
{
	ios::sync_with_stdio(false);
	cin.tie(nullptr);
	
	int n;
	cin >> n;
	double ans = 2*acos(-1)*n;
	cout << fixed << setprecision(20) ;
	cout << ans << '\n';
				
	return 0;
}
