#include <bits/stdc++.h>
using namespace std;
#define fastt ios_base::sync_with_stdio(false);cin.tie(NULL);
#define enl "\n"
#define ll long long
#define pb push_back
#define fi first
#define se second
#define vvl vector<vector<ll>> 
#define vl vector<ll> 
#define vpll vector<pair<ll,ll>>
#define all(x) x.begin(),x.end()
#define ffor(i,n) for(ll i = 0; i < n; ++i)
#define PI 3.14159265 
ll mod=1e9+7;

 int main(){
     fastt;
     ll i,j,k;
   cin>>j;
   cout<<2*PI*j<<enl;
 }
