#include<bits/stdc++.h>
#if __has_include(<atcoder/all>)
#endif
using namespace std;
#define RD(T,...) T __VA_ARGS__;lin(__VA_ARGS__)
#define LL(...) RD(ll,__VA_ARGS__)
#define DD(...) RD(dd,__VA_ARGS__)
#define RDVL(T,n,...) vec<T>__VA_ARGS__;fe(refs(__VA_ARGS__),e)e.get().resizes(n);lin(__VA_ARGS__)
#define VL(n,...) RDVL(ll,n,__VA_ARGS__)
#define FO(n) for(ll IJK=n;IJK-->0;)
#define fo(i,...) for(auto[i,i##stop,i##step]=for_range<ll>(0,__VA_ARGS__);i<i##stop;i+=i##step)
#define fe(a,e,...) for(auto&&__VA_OPT__([)e __VA_OPT__(,__VA_ARGS__]):a)
#define defpp template<ostream&o=cout>void pp(const auto&...a){[[maybe_unused]]const char*c="";((o<<c<<a,c=" "),...);o<<'\n';}void epp(const auto&...a){pp<cerr>(a...);}
#define entry defpp void main();void main2();}int main(){my::io();my::main();}namespace my{
#define multi_test_cases LL(T);FO(T)main2();}void main2(){
namespace my{
void io(){cin.tie(nullptr)->sync_with_stdio(0);cout<<fixed<<setprecision(15);}
using dd=double;
using ll=long long;
constexpr auto refs(auto&...a){return array{ref(a)...};}
template<class T>constexpr auto for_range(T s,T b){T a=0;if(s)swap(a,b);return array{a-s,b,1-s*2};}
template<class T>constexpr auto for_range(T s,T a,T b,T c=1){return array{a-s,b,(1-s*2)*c};}
void lin(auto&...a){(cin>>...>>a);}
constexpr ll size10(auto x){x|=1;ll r=0;while(x>0)x/=10,++r;return r;}
template<class T>constexpr ll maxsize10(){return size10(numeric_limits<T>::max());}
bool amin(auto&a,const auto&b){return b<a?a=b,1:0;}
template<bool is_negative=false>struct infinity{
  template<integral T>static constexpr T ones(size_t n){return n?ones<T>(n-1)*10+1:0;}
  template<integral T>constexpr operator T()const{return ones<T>(maxsize10<T>())*(1-is_negative*2);}
  template<floating_point T>constexpr operator T()const{return static_cast<ll>(*this);}
};
constexpr infinity oo;
template<class...A>using pack_back_t=tuple_element_t<sizeof...(A)-1,tuple<A...>>;
}
namespace my{
template<class V>istream&operator>>(istream&i,vector<V>&v){fe(v,e)i>>e;return i;}
template<class V>constexpr int depth=0;
template<class T>struct core_t_helper{using type=T;};
template<class T>using core_t=core_t_helper<T>::type;
template<class V>struct vec;
template<int D,class T>struct hvec_helper{using type=vec<typename hvec_helper<D-1,T>::type>;};
template<class T>struct hvec_helper<0,T>{using type=T;};
template<int D,class T>using hvec=hvec_helper<D,T>::type;
template<class V>struct vec:vector<V>{
  static constexpr int D=depth<V>+1;
  using C=core_t<V>;
  using vector<V>::vector;
  void resizes(const auto&...a){if constexpr(sizeof...(a)==D)*this=make(a...,C{});else{ }}
  static auto make(ll n,const auto&...a){
    if constexpr(sizeof...(a)==1)return vec<C>(n,array{a...}[0]);
    else { }
  }
  auto&emplace_front(auto&&...a){this->emplace(this->begin(),std::forward<decltype(a)>(a)...);return*this;}
  template<class T=C>auto min(ll l,ll r)const{T res=oo;fo(i,l,r)amin(res,(*this)[i]);return res;}
};
template<class...A>requires(sizeof...(A)>=2)vec(const A&...a)->vec<hvec<sizeof...(A)-2,pack_back_t<A...>>>;
}
namespace my{entry
void main(){
  multi_test_cases
  LL(N);
  DD(X);
  VL(N,h,a);
  h.emplace_front(0);
  auto calc=[&](ll k)->dd{
    if(a.min(0,k)<=a[k])return oo;
    dd res=0;
    dd su=0;

    fo(i,k){
      dd d=h[i+1]-h[i];
      dd th=acos(dd(a[k])/a[i]);
      res+=d/std::sin(th)*a[i];
      su+=d/tan(th);
    }
    if(X/2-su<0)return oo;
    res+=(X/2-su)*a[k];
    return res*2;
  };

  dd ans=oo;
  fo(i,N)amin(ans,calc(i));
  pp(ans);
}}