#include <iostream>
#include <string>
#include <cstring>
#include <cstdlib>
#include <cmath>
#include <algorithm>
#include <vector>
#include <set>
#include <map>
#include <queue>
#include <stack>
#include <list>
#include <iterator>
#include <cassert>
#include <numeric>
#include <functional>
#include <ctime>
#pragma warning(disable:4996) 

//#define ATCODER
#ifdef ATCODER
#include <atcoder/all>
#endif

typedef long long ll;
typedef unsigned long long ull;
#define LINF  9223300000000000000
#define LINF2 1223300000000000000
#define LINF3 1000000000000
#define INF 2140000000
//const long long MOD = 1000000007;
const long long MOD = 998244353;

using namespace std;
#ifdef ATCODER
using namespace atcoder;
#endif

void solve()
{
    int n, x;
    scanf("%d%d", &n, &x);
    vector<int> h(n), a(n), hdiff(n);
    int i, j;
    for (i = 0; i < n; i++) {
        scanf("%d", &h[i]);
        hdiff[i] = h[i] - (i==0? 0: h[i - 1]);
    }

    for (i = 0; i < n; i++) {
        scanf("%d", &a[i]);
    }

    double ans = (double)x * a[0];
    int mina = INF;
    for (i = 0; i < n; i++) {
        if (a[i] < mina) {
            double sumdist = 0;
            double sumtime = 0;
            for (j = 0; j < i; j++) {
                double sint = (double)a[i] / a[j];
                double t = asin(sint);
                double dist = tan(t) * hdiff[j];
                sumdist += dist;
                sumtime += (double)hdiff[j] / cos(t) * a[j] * 2;
            }
            if (sumdist * 2 < x) {
                ans = min(ans, sumtime + (x - sumdist * 2) * a[i]);
            }
        }
        mina = min(mina, a[i]);
    }
    printf("%.10lf\n", ans);

    return;
}

int main()
{
#if 0
    solve();
#else
    int T, t;
    scanf("%d", &T);
    for (t = 0; t < T; t++) {
        solve();
    }
#endif
    return 0;
}