#include <bits/stdc++.h>

using ll   = long long;
using uint = unsigned int;
using ull  = unsigned long long;
using ld   = long double;
template<typename T>
using max_heap = std::priority_queue<T>;
template<typename T>
using min_heap = std::priority_queue<T, std::vector<T>, std::greater<T>>;
constexpr int popcount(const ull v) { return v ? __builtin_popcountll(v) : 0; }
constexpr int log2p1(const ull v) { return v ? 64 - __builtin_clzll(v) : 0; }
constexpr int lsbp1(const ull v) { return __builtin_ffsll(v); }
constexpr int clog(const ull v) { return v ? log2p1(v - 1) : 0; }
constexpr ull ceil2(const ull v) { return 1ULL << clog(v); }
constexpr ull floor2(const ull v) { return v ? (1ULL << (log2p1(v) - 1)) : 0ULL; }
constexpr bool btest(const ull mask, const int ind) { return (mask >> ind) & 1ULL; }
template<typename T> void bset(T& mask, const int ind) { mask |= ((T)1 << ind); }
template<typename T> void breset(T& mask, const int ind) { mask &= ~((T)1 << ind); }
template<typename T> void bflip(T& mask, const int ind) { mask ^= ((T)1 << ind); }
template<typename T> void bset(T& mask, const int ind, const bool b) { (b ? bset(mask, ind) : breset(mask, ind)); }
template<typename T> bool chmin(T& a, const T& b) { return (a > b ? a = b, true : false); }
template<typename T> bool chmax(T& a, const T& b) { return (a < b ? a = b, true : false); }

template<typename T> constexpr T inf_v      = std::numeric_limits<T>::max() / 4;
template<typename Real> constexpr Real pi_v = Real{3.141592653589793238462643383279502884};
constexpr ull TEN(const int n) { return n == 0 ? 1ULL : TEN(n - 1) * 10ULL; }
template<typename F> struct fix : F
{
    fix(F&& f) : F{std::forward<F>(f)} {}
    template<typename... Args> auto operator()(Args&&... args) const { return F::operator()(*this, std::forward<Args>(args)...); }
};
class printer
{
public:
    printer(std::ostream& os_ = std::cout) : os{os_} {}
    template<typename T> int operator()(const T& v) { return os << v, 0; }
    template<typename T> int operator()(const std::vector<T>& vs)
    {
        for (int i = 0; i < (int)vs.size(); i++) { os << (i ? " " : ""), this->operator()(vs[i]); }
        return 0;
    }
    template<typename T> int operator()(const std::vector<std::vector<T>>& vss)
    {
        for (int i = 0; i < (int)vss.size(); i++) { os << (0 <= i or i + 1 < (int)vss.size() ? "\n" : ""), this->operator()(vss[i]); }
        return 0;
    }
    template<typename T, typename... Args> int operator()(const T& v, const Args&... args) { return this->operator()(v), os << ' ', this->operator()(args...), 0; }
    template<typename... Args> int ln(const Args&... args) { return this->operator()(args...), os << '\n', 0; }
    template<typename... Args> int el(const Args&... args) { return this->operator()(args...), os << std::endl, 0; }
    template<typename... Args> int fmt(const std::string& s, const Args&... args) { return rec(s, 0, args...); }

private:
    int rec(const std::string& s, int index) { return os << s.substr(index, s.size()), 0; }
    template<typename T, typename... Args> int rec(const std::string& s, int index, const T& v, const Args&... args) { return index == s.size() ? 0 : s[index] == '%' ? (this->operator()(v), rec(s, index + 1, args...)) : (os << s[index], rec(s, index + 1, v, args...)); }
    std::ostream& os;
};
printer out;

template<typename T, typename F> std::vector<T> generated(const int n, F f)
{
    std::vector<T> ans(n);
    return std::generate(ans.begin(), ans.end(), f), ans;
}
std::vector<int> ioted(const int n, const int offset = 0)
{
    std::vector<int> ans(n);
    return std::iota(ans.begin(), ans.end(), offset), ans;
}
template<typename Vs> Vs reversed(const Vs& vs)
{
    auto ans = vs;
    return std::reverse(ans.begin(), ans.end()), ans;
}
template<typename T, typename F = std::less<T>> std::vector<T> sorted(const std::vector<T>& vs, F comp = F{})
{
    auto ans = vs;
    return std::sort(ans.begin(), ans.end(), comp), ans;
}
template<typename F> std::vector<int> sorted_iota(const int n, F comp = F{}, const int offset = 0) { return sorted(ioted(n, offset), comp); }
class scanner
{
public:
    scanner(std::istream& is_ = std::cin) : is{is_} { is.tie(nullptr), std::ios::sync_with_stdio(false); }
    template<typename T> T val()
    {
        static T v;
        return is >> v, v;
    }
    template<typename T> T val(const T offset) { return val<T>() - offset; }
    template<typename T> std::vector<T> vec(const int n)
    {
        return generated<T>(n, [&] { return val<T>(); });
    }
    template<typename T> std::vector<T> vec(const int n, const T offset)
    {
        return generated<T>(n, [&] { return val<T>(offset); });
    }
    template<typename T> std::vector<std::vector<T>> vvec(const int n0, const int n1)
    {
        return generated<std::vector<T>>(n0, [&] { return vec<T>(n1); });
    }
    template<typename T> std::vector<std::vector<T>> vvec(const int n0, const int n1, const T offset)
    {
        return generated<std::vector<T>>(n0, [&] { return vec<T>(n1, offset); });
    }
    template<typename... Args> auto tup() { return std::tuple<std::decay_t<Args>...>{val<Args>()...}; }
    template<typename... Args> auto tup(const Args&... offsets) { return std::tuple<std::decay_t<Args>...>{val<Args>(offsets)...}; }

private:
    std::istream& is;
};
scanner in;
#    define SHOW(...) static_cast<void>(0)

template<typename T, int n, int i = 0>
auto make_v(int const (&szs)[n], const T x = T{})
{
    if constexpr (i == n) {
        return x;
    } else {
        return std::vector(szs[i], make_v<T, n, i + 1>(szs, x));
    }
}


template<typename T = int>
struct edge
{
    using cost_type = T;
    int u, v;
    T c;
    edge(const int u_, const int v_) : u{u_}, v{v_}, c{1} {}
    edge(const int u_, const int v_, const T& c_) : u{u_}, v{v_}, c{c_} {}
    operator int() const { return v; }
    int from() const { return u; }
    int to() const { return v; }
    T cost() const { return c; }
    friend std::ostream& operator<<(std::ostream& os, const edge& e) { return os << e.u << "->" << e.v << ":" << e.c; }
};
template<typename Edge>
class base_graph
{
public:
    base_graph(const int n) : sz{n}, es(n), res(n) {}
    void add_edge(const int u, const int v, const bool bi = false)
    {
        es[u].emplace_back(u, v), res[v].emplace_back(v, u);
        if (bi) { es[v].emplace_back(v, u), res[u].emplace_back(u, v); }
    }
    template<typename Cost>
    void add_edge(const int u, const int v, const Cost& c, const bool bi = false)
    {
        es[u].emplace_back(u, v, c), res[v].emplace_back(v, u, c);
        if (bi) { es[v].emplace_back(v, u, c), res[u].emplace_back(u, v, c); }
    }
    std::vector<Edge>& operator[](const int u) { return es[u]; }
    const std::vector<Edge>& operator[](const int u) const { return es[u]; }
    std::vector<Edge>& from(const int u) { return es[u]; }
    const std::vector<Edge>& from(const int u) const { return es[u]; }
    std::vector<Edge>& to(const int v) { return res[v]; }
    const std::vector<Edge>& to(const int v) const { return res[v]; }
    int size() const { return sz; }
    friend std::ostream& operator<<(std::ostream& os, const base_graph& g)
    {
        for (int i = 0; i < g.sz; i++) {
            for (const auto& e : g.es[i]) { os << e << '\n'; }
        }
        return os;
    }

private:
    int sz;
    std::vector<std::vector<Edge>> es, res;
};
template<typename Edge>
using base_tree = base_graph<Edge>;
using graph     = base_graph<edge<>>;
using tree      = base_graph<edge<>>;
template<typename Cost>
using cost_graph = base_graph<edge<Cost>>;
template<typename Cost>
using cost_tree = base_graph<edge<Cost>>;
template<typename Edge>
std::vector<typename Edge::cost_type> dijkstra(const base_graph<Edge>& g, const int s)
{
    using T = typename Edge::cost_type;
    std::vector<T> d(g.size(), inf_v<T>);
    using P = std::pair<T, int>;
    std::priority_queue<P, std::vector<P>, std::greater<P>> q;
    d[s] = 0, q.push({0, s});
    while (not q.empty()) {
        const T cost = q.top().first;
        const int v  = q.top().second;
        q.pop();
        if (d[v] < cost) { continue; }
        for (const auto& e : g[v]) {
            const auto c = e.cost();
            const int to = e.to();
            if (d[to] <= d[v] + c) { continue; }
            d[to] = d[v] + c, q.push({d[to], to});
        }
    }
    return d;
}
int main()
{
    const auto [N, K] = in.tup<int, ll>();
    const auto as     = in.vec<ll>(N);
    const auto bs     = in.vec<ll>(N);
    cost_graph<ll> g(2 * N);
    for (int i = 0; i < N; i++) { g.add_edge(i, i + N, as[i]); }
    for (int i = 0; i + 1 < N; i++) { g.add_edge(i + N, i + 1, bs[i + 1]); }
    for (int i = 0; i + 2 < N; i++) { g.add_edge(i + N, i + 2, bs[i + 2] + K); }
    const auto ts = dijkstra(g, 0);
    ll ans        = 0;
    for (int i = 0; i < N; i++) { chmax(ans, ts[i]); }
    out.ln(ans);
    return 0;
}