#define _USE_MATH_DEFINES
#include <bits/stdc++.h>
using namespace std;
#define FOR(i,m,n) for(int i=(m);i<(n);++i)
#define REP(i,n) FOR(i,0,n)
#define ALL(v) (v).begin(),(v).end()
using ll = long long;
constexpr int INF = 0x3f3f3f3f;
constexpr long long LINF = 0x3f3f3f3f3f3f3f3fLL;
constexpr double EPS = 1e-8;
constexpr int MOD = 1000000007;
// constexpr int MOD = 998244353;
constexpr int DY4[]{1, 0, -1, 0}, DX4[]{0, -1, 0, 1};
constexpr int DY8[]{1, 1, 0, -1, -1, -1, 0, 1};
constexpr int DX8[]{0, -1, -1, -1, 0, 1, 1, 1};
template <typename T, typename U>
inline bool chmax(T& a, U b) { return a < b ? (a = b, true) : false; }
template <typename T, typename U>
inline bool chmin(T& a, U b) { return a > b ? (a = b, true) : false; }
struct IOSetup {
  IOSetup() {
    std::cin.tie(nullptr);
    std::ios_base::sync_with_stdio(false);
    std::cout << fixed << setprecision(20);
  }
} iosetup;

void solve() {
  int n, x; cin >> n >> x;
  vector<int> h(n), a(n);
  REP(i, n) cin >> h[i];
  REP(i, n) cin >> a[i];
  double ans = 1LL * a.front() * x;
  int mini = a.front();
  FOR(i, 1, n) {
    if (!chmin(mini, a[i])) continue;
    double t = 0, len = 0;
    REP(j, i) {
      const int l = h[j] - (j == 0 ? 0 : h[j - 1]);
      double lb = 0, ub = x;
      const auto f = [&](double x) -> double {
        return sqrt(x * x + 1LL * l * l);
      };
      REP(_, 80) {
        const double m1 = (lb + lb + ub) / 3, m2 = (lb + ub + ub) / 3;
        if (a[j] * f(m1) + a[i] * (m2 - m1) < a[j] * f(m2)) {
          ub = m2;
        } else {
          lb = m1;
        }
      }
      len += lb * 2;
      t += a[j] * f(lb) * 2;
    }
    if (len <= x) chmin(ans, t + (x - len) * a[i]);
  }
  cout << ans << '\n';
}

int main() {
  int t; cin >> t;
  while (t--) solve();
  return 0;
}