/*---------------------------------------------------------------------*/
/*                   -------------------------------------             */
/*                   -----------MEHEDI HASAN--------------             */
/*                   -----CSE,JAHANGIRNAGAR UNIVERSITY----             */
/*                   -------------------------------------             */
/*---------------------------------------------------------------------*/
#include<bits/stdc++.h>
//#include <cassert>
//#include <ext/pb_ds/assoc_container.hpp>
//#include <ext/pb_ds/tree_policy.hpp>
//using namespace __gnu_pbds;
using namespace std;
/*---------------------------------------------------------------------*/
#define FAST        ios_base::sync_with_stdio(NULL);cin.tie(NULL);cout.tie(NULL);
#define cc(x)       cout<<#x<<"="<<x<<" ";
#define ccc(x)      cout<<#x<<"="<<"("<<x.ff<<" "<<x.ss<<") ";
#define CC(x)       cout<<"["<<#x<<"] = ";for(auto xx: x) cout<<xx<<" ";
#define CCC(x)      cout<<"["<<#x<<"] = ";for(auto xx: x) cout<<"("<<xx.ff<<" "<<xx.ss<<") ";
#define MARK(x)     cout<<x<<" ";
#define ee          cout<<"\n";
#define OK          cout<<"Ok till line "<<__LINE__<<"\n";
/*---------------------------------------------------------------------*/
#define ff          first
#define ss          second
#define ll          long long
#define ull         unsigned long long
#define pii         pair<int,int>
#define mp          make_pair
#define ps          push
#define psb         push_back
#define psf         push_front
#define pp          pop
#define ppb         pop_back
#define ppf         pop_front
#define all(x)      x.begin(),x.end()
#define TESTS       int t;cin>>t;for(int tcase=1;tcase<=t;tcase++)
#define ordered_set tree< int, null_type, less_equal< int >, rb_tree_tag, tree_order_statistics_node_update>
/*---------------------------------------------------------------------*/
#define MAX (int) (5e5+88)

int main()
{
    FAST
    int r;cin>>r;
    cout<<2*3.1416*r<<endl;
    return 0;
}
/*--------------------------------------------------------------------*/
/*
>Check Corner Cases (n==1 or T==1)
>Overflow issues?
>LB / UB !!not found!! check?
*/
/*--------------------------------------------------------------------*/
/* ---INPUT---
--------(1)------
4
3
AAB
5
AABAA
1
B
4
BABA
-------(2)-------
5
5
AAAAA
6
BBAAAA
1
A
2
BA
10
AAAAAAAAAB


-------(3)-------


-------(4)-------



--------------
*/
/*--------------------------------------------------------------------*/


//https://codeforces.com/contest/466/problem/C try again
