#include<bits/stdc++.h>
using namespace std;
typedef long long int ll;
//////////////////////////////////////////////////////////////////////////////////////////////////////////////////
#define  boost ios_base::sync_with_stdio(false);cin.tie(NULL);
#define ull unsigned long long
#define d1(x) cout<<#x<<" "<<x<<endl;
#define d2(x,y) cout<<#x<<" "<<x<<" "<<#y<<" "<<y<<endl;
#define d2i(x,y,i) cout<<#x<<i<<" "<<x<<" "<<#y<<i<<" "<<y<<endl;

#define fr(i,l,r) for(ll i=l;i<r;i++)
#define mems(a,x) memset(a,x,sizeof(a))
#define mod 1000000007
#define ff first
#define ss second
#define pb(x) push_back(x)
#define vll vector<ll>
#define pbp(x,y) push_back(make_pair(x,y))
#define  mat vector<vector<ll>>
#define pi 3.14159265358979323846
////////////////////////////////////////////////////////////////////////////////////////////////////////////////////

void solve(){
  double r;
  cin>>r;
 cout<<fixed;
 cout<<setprecision(10)<<(double)2*pi*r<<endl;
}





int main(){
    ll t=1;
//cin>>t;
    while(t--){
     solve();
   cout<<endl;
    }
    return 0;
}
