// IO library
#include <cstdio>
#include <iomanip>
#include <ios>
#include <iostream>

// algorithm library
#include <algorithm>
#include <cmath>
#include <numeric>
#include <random>

// contancer library
#include <bitset>
#include <deque>
#include <map>
#include <queue>
#include <set>
#include <string>
#include <tuple>
#include <vector>
#include <functional>
#include <limits>

using ll = long long;
using ld = long double;

/* ----- Output Functions for Debugging ----- */

template <class T>
std::ostream& operator<<(std::ostream& os, std::vector<T> v);
template <class T>
std::ostream& operator<<(std::ostream& os, std::set<T> v);
template <class L, class R>
std::ostream& operator<<(std::ostream& os, std::pair<L, R> p);
template <class K, class T>
std::ostream& operator<<(std::ostream& os, std::map<K, T> v);
template <class T>
std::ostream& operator<<(std::ostream& os, std::queue<T> q);
template <class T>
std::ostream& operator<<(std::ostream& os, std::priority_queue<T> q);

template <class T>
std::ostream& operator<<(std::ostream& os, std::vector<T> v) {
    os << "[";
    for (auto vv : v) os << vv << ",";
    return os << "]";
}

template <class T>
std::ostream& operator<<(std::ostream& os, std::set<T> v) {
    os << "{";
    for (auto vv : v) os << vv << ",";
    return os << "}";
}

template <class L, class R>
std::ostream& operator<<(std::ostream& os, std::pair<L, R> p) {
    return os << "(" << p.first << "," << p.second << ")";
}

template <class K, class T>
std::ostream& operator<<(std::ostream& os, std::map<K, T> v) {
    os << "{";
    for (auto vv : v) os << vv << ",";
    return os << "}";
}

template <class T>
std::ostream& operator<<(std::ostream& os, std::queue<T> q) {
    os << "[";
    while (!q.empty()) {
        os << q.front() << ",";
        q.pop();
    }
    return os << "]";
}

template <class T>
std::ostream& operator<<(std::ostream& os, std::priority_queue<T> q) {
    os << "{";
    while (!q.empty()) {
        os << q.top() << ",";
        q.pop();
    }
    return os << "}";
}

/* ----- Short Functions ----- */

template <class T>
T Vec(T v) { return v; }

template <class T, class... Ts>
auto Vec(size_t l, Ts... ts) {
    return std::vector<decltype(Vec<T>(ts...))>(l, Vec<T>(ts...));
}

template <class T>
inline T sq(T a) { return a * a; }

template <class T>
inline T iceil(T n, T d) { return (n + d - 1) / d; }

template <class T>
T gcd(T a, T b) {
    while (b > 0) {
        a %= b;
        swap(a, b);
    }
    return a;
}

template <class T, class U>
T ipow(T b, U n) {
    T ret = 1;
    while (n > 0) {
        if (n & 1) ret *= b;
        n >>= 1;
        b *= b;
    }
    return ret;
}

// 0-indexed
template <class T, class U>
inline T kthbit(T a, U k) { return (a >> k) & 1; }

template <class T, class U>
inline T mask(T a, U k) { return a & ((1 << k) - 1); }

/* ----- class ----- */

template <class T>
struct Edge {
    int from, to;
    T cost;
    Edge(int from = -1, int to = -1, T cost = 1) : from(from), to(to), cost(cost){};

    bool operator<(const Edge<T>& e) const { return this->cost < e.cost; }
    bool operator>(const Edge<T>& e) const { return this->cost > e.cost; }
};

template <class T = int>
class Graph {
public:
    explicit Graph(int N = 0) : size(N) { path.resize(size); }
    void span(int u, int v, T cost = 1) { path[u].push_back(Edge<T>(u, v, cost)); }
    std::vector<Edge<T>> operator[](int v) const { return path[v]; }

    int size;
    std::vector<std::vector<Edge<T>>> path;
};

/* ----- Constants ----- */

// const int INF = 1 << 25;
// const ll INF = 1LL << 50;
// const ld PI = acos(-1);
// const ld EPS = 1e-10;
// mt19937 mt(ll(time(0)));

int main() {
    int N;
    ll L;
    std::cin >> N >> L;

    std::vector<std::pair<ll, int>> pos(N * 2);
    for (int i = 0; i < N; ++i) {
        std::cin >> pos[i].first;
        pos[i].second = 1;
    }
    for (int i = 0; i < N; ++i) {
        std::cin >> pos[i + N].first;
        pos[i + N].second = -1;
    }

    std::sort(pos.begin(), pos.end());
    pos.push_back(pos.front());

    std::vector<ll> sushi(N * 2 + 1);
    sushi[0] = 0;
    for (int i = 0; i < N * 2; ++i) {
        sushi[i + 1] = sushi[i] + pos[i].second;
    }
    ll min = *std::min_element(sushi.begin(), sushi.end());
    if (min < 0) std::terminate();
    for (auto& s : sushi) s -= min;

    ll ans = 0;
    for (int i = 1; i <= N * 2; ++i) {
        if (pos[i].second < 0) {
            ans = std::max(ans, pos[i].first + L * (sushi[i] - 1));
        }
    }
    std::cout << ans << std::endl;
    return 0;
}