#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 int
#define fo(i,n) for(i=0;i<n;i++)
#define pb push_back
#define mp make_pair
#define endl "\n"
#define MAXN 1000002
typedef pair<int,int> pii;
typedef pair<ll,ll> pll;
typedef vector<int> vi;
typedef vector<ll> vll;
typedef vector<pii> vpii;
typedef vector<pll> vpll;
const ll mod=1e9+7;

void solve()
{
  double pi=3.14159;
  double n;
  cin>>n;
  double a=2;
  double ans=a*pi*n;
  cout<<setprecision(6)<<fixed<<ans;
}

int main()
{
  fastio
  solve();
}
