    #include"bits/stdc++.h"
    using namespace std;
    using ll = long long ;
    ///PBDS Starting.......

    #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>

    ///PBDS_SET
    ///Index of Value - .order_of_key()
    ///Value at Index - .find_by_order()
    ///typedef tree<int, null_type, less<int>, rb_tree_tag,tree_order_statistics_node_update> pbds_set;
    ///typedef tree<int, null_type, lessorequal<int>, rb_tree_tag,tree_order_statistics_node_update> pbds_multiset;

    /// PBDS closing......
    #define int long long
    #define FAST_IO ios_base::sync_with_stdio(false); cin.tie(nullptr); cout.tie(nullptr); cout.setf(ios::fixed); cout.precision(20)
    #define endl '\n'
    ///............Author..................................................................
    ///........... :: .....................................................................
    ///...........(  NOBODY  )................................................................
    ///........... Bangabandhu Sheikh Mujibur Rahman Science and Technlogy University,Gopalganj
    ///........... ( N~76 ) ...................................................................
    ///........... No Copyright .....You can copy and paste my code without any permission.....
    /// !!!!!!!!!!!!!!!........................................................................
    #define pb push_back
    #define esp 0.000000001
    #define pi acos(-1)
    #define N 1000009
    #define ff first
    #define ss second
    #define all(x) x.begin(),x.end()
    /// Code Loading Please wait.............
    /// returning to N~76.
    /// Loading Finished.....................

    int32_t main()
    {
        FAST_IO;
        int n;
        cin>>n;
        double ans=(double)2*pi*n;
        cout<<ans<<endl;
        return 0;
    }

