#include<bits/stdc++.h>
using namespace std;
#define rep(i,n) for(int i=0;i<n;i++)
#define repA(i,a,n) for(int i=a;i<n;i++)
#define trav(a,x)for(auto &a: x)
#define fll(a) memset(a,0,sizeof(a));
#define mp make_pair
#define pb push_back
#define all(x) x.begin(),x.end()
#define F first
#define S second
#define itr(it,x) for(auto it=x.begin();it!=x.end();it++)
typedef long long ll;
typedef long double ld;
typedef pair<int,int>pii;
typedef vector<int>vi;
typedef map<int,int>ma;
#define fastio ios_base::sync_with_stdio(false); cin.tie(NULL);
void fopen()
{
    freopen("input.txt","r",stdin);
    freopen("output.txt","w",stdout);   
}
# define PI    3.14159265358979323846  
void solve()
{
  ld r;
  cin>>r;
  cout<<2*PI*r;

}
int main()
{
  fastio;
 // fopen();
  int t;
//  cin>>t;
  rep(i,1)
  { 
   // cout<<"Case #"<<i+1<<": ";
    solve();
    cout<<endl;
  }
  
}