#include <bits/stdc++.h>
using namespace std;
typedef pair<int,int> pii;
 
#define mem(dp,a)       memset(dp,a,sizeof dp)
#define ninja           ios_base::sync_with_stdio(false); cin.tie(NULL); cout.tie(NULL);
#define pb(x)           push_back(x)
#define mp(x,y)         make_pair(x,y)
#define all(c)          c.begin(), c.end()
#define int             long long
#define ff              first
#define ss              second
#define nl              '\n'
#define inf             1e9+7

signed main()
{
	ninja
	int t; 
	t=1;
	while(t--)
	{
		float r; cin>>r;
		float pi=3.141592653589793238;
		cout<<fixed<<setprecision(9)<< 2*pi*r;
	}
}

