//
// Created by kinok on 2020/04/19.
//

#include <iostream>
#include <vector>
#include <algorithm>
#include <cmath>
#include <string>
#include <iomanip>

using namespace std;

#ifndef UNTITLED2_ATCODER163_H
#define UNTITLED2_ATCODER163_H

#define ll long long
#define rep(t) for(int i=0;i<t;i++)
#define deb(a) cout<<#a<<" : "<<a<<endl
#define debv(v) do{cout<<#v<<" : ";for(auto a:v){cout << a <<" ";} cout<<endl;}while(0);
#define PI 3.141592653589793

class atcoder163{
public:void main(){
    a();
}

void a(){
    int a;
    cin>>a;
    cout<<fixed<<setprecision(8)<<a*2*PI<<endl;
}

void b(){
    ll n,m,total=0,k;
    cin>>n>>m;
    rep(m){
        cin>>k;
        total+=k;
    }
    cout<<(n<total?-1:n-total)<<endl;
}
void c(){

}

void d(){

}

void e(){

}

void f(){

}

};


#endif //UNTITLED2_ATCODER163_H
