#include <algorithm>
#include<bits/stdc++.h>
#include <ios>
#include <iterator>
using namespace std;
#define ll long long
#define f(n)  for(int i=0;i<n;i++) 
#define pb push_back
#define mp make_pair
#define vi vector< int>
#define all(v)   v.begin(),v.end()  
#define take(n)   ll int n;cin>>n;
void speed (){
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);
    cout.tie(NULL);
}
 
int main() {
	speed();
 int r;
    cin>>r;
    double ans=6.28318530717958623200*r;
    cout.precision(20);
    cout<<fixed<<ans;
	return 0;
}

 