#include<bits/stdc++.h>
#define ll long long int 
#define mod 1000000007
#define se second
#define fi first
#define endl "\n"
#define pi 3.1415926535
using namespace std;
int main()
{
	ios_base::sync_with_stdio(false);
	cin.tie(0);cout.tie(0);
	double r;
	cin>>r;
	cout<<fixed<<setprecision(10);
	cout<<2*pi*r;
	return 0;
}
