#include <bits/stdc++.h>
using namespace std;

int const MOD = 1e9+5;

#define  pi acos(-1.00)
#define fastIO ios_base::sync_with_stdio(false);cin.tie(NULL);cout.tie(NULL)
#define newLine printf("\n")
#define start_p int main()
#define end_p return 0
#define test_IO freopen("input.txt", "r", stdin); freopen("output.txt", "w", stdout)
#define pb push_back

typedef long long ll;
typedef vector<int> vi;
typedef vector<char> vc;


start_p{
    #ifndef ONLINE_JUDGE
    //test_IO;
    #endif // ONLINE_JUDGE

    double r; cin>>r;
    double c = 2*pi*r;
    cout<<setprecision(20)<<fixed<<c<<endl;
    //67252742410977361942
    end_p;
}

