#include<bits/stdc++.h>
using namespace std;
#define rep(i,a) for(int i=0;i<a;i++)
#define pd(a,n) for(int i=0;i<n;i++)cout<<a[i]<<((i==n-1)?"\n":" ");cout<<endl;
#define pdd(a,n,m)for(int i=0;i<n;i++){for(int j=0;j<m;j++)cout<<a[i][j]<<" ";cout<<endl;}
#define mp make_pair
#define pb push_back
#define ll long long
//#define int ll
//#define __int64 lng long
#define P pair<int,int>
#define PP pair<P,int>

double r;

signed main(){
  cin>>r;
  printf("%.10lf",2.0*3.1415926535*r);
  return 0;
}
