#include<bits/stdc++.h>
using namespace std;

typedef long long ll;
typedef vector<ll> vi;
typedef pair<ll, ll> ii;
typedef vector<ii> vii;
typedef set<ll> si;
typedef map<string, ll> msi;
#define hell 1000000007
#define MAX 1000000000
#define max_int 1000000
#define F first
#define re 15000000
#define S second
#define pb push_back
#define all(a)  (a).begin(),(a).end()
#define rep(i,a,b) for(ll i = a;i<b;i++)
#define pi 3.14159265358979311600
#define r0 return 0

int main()
{
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);
    cout.tie(NULL);
 
    //freopen("input.txt", "r", stdin);
    //freopen("output.txt", "w", stdout);
    long double n;
    cin>>n;
    cout<<setprecision(20)<<fixed<<2*pi*n;
  
 
    
}
 