	#include<bits/stdc++.h>
	using namespace std;
	#define fo(i,n) for(int i=0;i<n;i++)
	#define pb push_back
	#define mp map<int,int>
	#define ump unordered_map<int,int>
	#define vi vector<int>
	#define M INT_MAX
	#define MI INT_MIN
	#define ll long long int
	#define pii pair<int,int>
	#define F first
	#define S second
	#define fall(i,a,b) for(int i=a;i<b;i++)
	#define pi 3.1415926535
	void solve()
	{
			int r;
			cin>>r;
			cout<<(2*pi*r)<<endl;
	}
	int main()
	{
		ios_base::sync_with_stdio(false);
	    cin.tie(NULL);
		solve();
	}