#include<bits/stdc++.h>
using namespace std;
#include <ext/pb_ds/assoc_container.hpp> 
#include <ext/pb_ds/tree_policy.hpp> 
using namespace __gnu_pbds; 
#define ordered_set tree<int, null_type,less<int>, rb_tree_tag,tree_order_statistics_node_update> 
#define ll long long
#define ld long double
#define MAX 1000000000
int main(){
	ios_base::sync_with_stdio(false);
    cin.tie(NULL);
	int t=1;
	//cin>>t;
	for(int cc=1;cc<=t;cc++){
        //cout<<"Case #"<<cc<<": ";
        ld r;
        cin>>r;
        cout<<(2*M_PI*r)<<endl;

	}
}
