#include<bits/stdc++.h>
using namespace std;
#define int long long int 
#define pb push_back
#define mp make_pair
#define fi first
#define se second
#define vi vector<int>
#define pi pair<int,int>
#define all(v) v.begin(),v.end()
#define zeros(arr) memset(arr,0,sizeof(arr));
#define precise(num)  cout<<fixed; cout << std::setprecision(num)
#define nl cout<<"\n";
int32_t main(){
    // boost;
    double pie ;
    pie =  3.141592653;
   
    double r;
    cin>>r;
    double ans = 2*pie*r;
    precise(10);
    cout<<ans<<endl;
    
}