#include<bits/stdc++.h>
using namespace std;
#define FASTIO ios_base::sync_with_stdio(false);cin.tie(NULL);cout.tie(NULL);
#define ll long long
#define pii pair<int,int>
#define pll pair<ll,ll>
#define pil pair<int,ll>
#define pli pair<ll,int>
#define mod 1000000000
#define mp make_pair
#define pb push_back
#define ff first
#define ss second
#define pi 3.14159
ll power(ll a,int n){
	if(n==0)
	  return 1;
	if((n%2)==0)
	  return power(a*a,n/2);
	else
	  return a*power(a*a,n/2);    
}
int main(){
FASTIO
double r;
cin>>r;
cout<<2*pi*r;
return 0;
}
