import sys
read = sys.stdin.buffer.read
readline = sys.stdin.buffer.readline
readlines = sys.stdin.buffer.readlines
from bisect import bisect


class BinaryIndexedTree():
    def __init__(self, seq):
        self.size = len(seq)
        self.depth = self.size.bit_length()
        self.build(seq)

    def build(self, seq):
        data = seq
        size = self.size
        for i, x in enumerate(data):
            j = i + (i & (-i))
            if j < size:
                data[j] += data[i]
        self.data = data

    def __repr__(self):
        return self.data.__repr__()

    def get_sum(self, i):
        data = self.data
        s = 0
        while i:
            s += data[i]
            i -= i & -i
        return s

    def add(self, i, x):
        data = self.data
        size = self.size
        while i < size:
            data[i] += x
            i += i & -i

    def find_kth_element(self, k):
        data = self.data
        size = self.size
        x, sx = 0, 0
        dx = 1 << (self.depth)
        for i in range(self.depth - 1, -1, -1):
            dx = (1 << i)
            if x + dx >= size:
                continue
            y = x + dx
            sy = sx + data[y]
            if sy < k:
                x, sx = y, sy
        return x + 1


N, L = map(int, readline().split())
X = tuple(map(int, readline().split()))
Y = tuple(map(int, readline().split()))

sushi = BinaryIndexedTree([0] + [1] * (N + N))
tea = BinaryIndexedTree([0] + [1] * (N + N))

x = X[0]
sushi.add(1, -1)
sushi.add(N + 1, -1)
cycle = 0

for _ in range(N - 1):
    # 次の茶まで進む
    k = tea.get_sum(bisect(Y, x))  # 左にある茶の個数
    i = tea.find_kth_element(k + 1)
    if i > N:
        i -= N
        cycle += 1
    x = Y[i - 1]
    # 次の寿司まで進む
    k = sushi.get_sum(bisect(X, x))
    i = sushi.find_kth_element(k + 1)
    if i > N:
        i -= N
        cycle += 1
    x = X[i - 1]
    # 寿司を削除
    sushi.add(i, -1)
    sushi.add(i + N, -1)
    # 手前のお茶を削除
    k = tea.get_sum(bisect(Y, x) + N)
    i = tea.find_kth_element(k)
    if i > N:
        i -= N
    tea.add(i, -1)
    tea.add(i + N, -1)

i = tea.find_kth_element(1)
y = Y[i - 1]
if x > y:
    y += L
print(cycle * L + y)