#include <iostream>
#include <iomanip>
#include <vector>
#include <set>
#include <unordered_set>
#include <map>
#include <unordered_map>
#include <queue>
#include <deque>
#include <stack>
#include <algorithm>
#include <numeric>
#include <string>
#include <sstream>
#include <cmath>
#include <cstring>
#include <assert.h>
#include <utility>
#include <tuple>
#include <array>
#include <bitset>
#include <cstdlib>

using int64 = long long;

template <typename T = int>
struct Range {
    struct RangeIterator {
        T i, step;

        RangeIterator(T i, T step): i{i}, step{step} {}

        T& operator*() { return i; }
        RangeIterator& operator++() {
            i += step;
            return *this;
        }
        bool operator!=(const RangeIterator& rhs) const {
            return (step > 0 && i < rhs.i) || (step < 0 && i > rhs.i);
        }
    };

    T start, stop, step;

    Range(T start, T stop, T step): start{start}, stop{stop}, step{step} {}
    Range(T start, T stop): Range(start, stop, 1) {}
    Range(T stop): Range(0, stop, 1) {}
    
    RangeIterator begin() const { return RangeIterator(start, step); }
    RangeIterator end() const { return RangeIterator(stop, step); }
};

struct Item {
    int x;
    int type;

    Item(int x, int type): x{x}, type{type} {}

    bool operator<(const Item& rhs) const {
        return x < rhs.x;
    }
};

static int N;
static int64 L;

int64 solveSpecialProblem(std::vector<Item>& items) {
    std::vector<int> d(items.size(), 0);

    for (const int i : Range<>(items.size())) {
        d[i] += items[i].type;
        if (i > 0) d[i] += d[i - 1];
    }

    int m = *std::max_element(d.begin(), d.end());
    int p = std::max_element(d.rbegin(), d.rend()) - d.rbegin();

    return (m - 1) * L + items[items.size() - p].x;
}

int64 solveProblem(std::vector<Item>& items) {
    std::vector<int> d(items.size(), 0);

    for (const int i : Range<>(items.size())) {
        d[i] += items[i].type;
        if (i > 0) d[i] += d[i - 1];
    }

    auto itr = std::min_element(d.begin(), d.end());

    int64 res = 0;

    if (*itr < 0) {
        int pos = itr - d.begin();
        res = items[pos + 1].x;

        // [0, pos)に寿司があれば
        std::vector<Item> new_items(items.begin() + pos + 1, items.end());

        bool sushi = false;
        for (const int i : Range<>(pos)) {
            if (!sushi && items[i].type == 1) {
                sushi = true;
            } else {
                new_items.push_back(items[i]);
            }
        }
        if (!sushi) new_items.push_back(items[pos]);

        // new_itemsの距離をresを用いて更新
        for (Item& item : new_items) {
            item.x = (item.x - items[pos + 1].x + L) % L;
        }
        items = new_items;
    }
    res += solveSpecialProblem(items);
    return res;
}

int main() {
    std::cin.tie(nullptr);
    std::ios::sync_with_stdio(false);

    std::cin >> N >> L;

    std::vector<Item> items;

    for (const int i : Range<>(N)) {
        int x;
        std::cin >> x;
        items.emplace_back(x, 1);
    }

    for (const int i : Range<>(N)) {
        int x;
        std::cin >> x;
        items.emplace_back(x, -1);
    }

    std::sort(items.begin(), items.end());

    std::cout << solveProblem(items) << std::endl;

    return 0;
}