#include <bits/stdc++.h>
using namespace std;
#define ll long long int
#include <ext/pb_ds/assoc_container.hpp> 
#include <ext/pb_ds/tree_policy.hpp> 
using namespace __gnu_pbds;   
#define ordered_set tree<ll, null_type,less<ll>, rb_tree_tag,tree_order_statistics_node_update> 
#define ld long double
#define ff first
#define ss second
#define pb push_back
#define mp make_pair
#define all(a) a.begin(),a.end()
#define sz(a) (ll)(a.size())
int main()
{
	ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0);
	int r;
	cin>>r;
	ld Radius = r;
	ld pi = 3.14159265359;
	cout<<fixed<<setprecision(8)<<2*pi*Radius<<"\n";
	return 0;
}