#include <bits/stdc++.h>
using namespace std;
typedef long long int ll;
typedef pair<ll,ll> P;
typedef vector<ll> VI;
typedef vector<VI> VVI;
#define REP(i,n) for(ll i=0;i<(n);i++)
#define ALL(v) v.begin(),v.end()
template<typename T> bool chmax(T &x, const T &y) {return (x<y)?(x=y,true):false;};
template<typename T> bool chmin(T &x, const T &y) {return (x>y)?(x=y,true):false;};
constexpr ll MOD=1000000007;
constexpr ll INF=2e18;

const double eps=1e-8;

double f(ll h, ll a, ll b){
    double l=0, r=1e9;
    double m1, m2, t1, t2;
    while(r-l>eps){
        m1=l+(r-l)/3;
        m2=l+(r-l)*2/3;
        t1=sqrt(m1*m1+h*h)*a-m1*b;
        t2=sqrt(m2*m2+h*h)*a-m2*b;
        if(t1<t2)
            r=m2;
        else
            l=m1;
    }
    return m1;
}

void solve(){
    ll n, x; cin >> n >> x;
    VI h(n+1), a(n); h[0]=0;
    REP(i,n) cin >> h[i+1];
    REP(i,n) cin >> a[i];
    double ans=INF;
    REP(i,n){
        double d=0, msum=0;
        for(int j=i-1;j>=0;j--){
            double res=f(h[j+1]-h[j],a[j],a[i]);
            msum+=res;
            d+=sqrt(res*res+(h[j+1]-h[j])*(h[j+1]-h[j]))*a[j];
        }
        if(msum*2>x)
            continue;
        d*=2;
        d+=(x-msum*2)*a[i];
        chmin(ans,d);
    }
    cout << setprecision(10) << ans << endl;
}

int main(){
    int t; cin >> t;
    while(t--) solve();
}