#include<bits/stdc++.h>
#define int long long
#define fi first
#define se second
#define pii pair<int,int>
#define mp make_pair
using namespace std;


main()
{
	ios_base::sync_with_stdio(false);
	cout.tie(nullptr);
	long double r;
	cin>>r;
	cout<<fixed<<setprecision(5)<<r*2*3.141592653;
}
