#include<cstddef>
#include<cstdint>
#include<iostream>
#include<limits>
#include<vector>

using isize = std::ptrdiff_t;
using usize = std::size_t;
using u64 = std::uint_fast64_t;

int main() {
	usize N;
	std::cin >> N;
	u64 L;
	std::cin >> L;
	std::vector<u64> x(N);
	for (auto &e : x) {
		std::cin >> e;
	}
	std::vector<u64> y(N);
	for (auto &e : y) {
		std::cin >> e;
	}

	x.emplace_back(std::numeric_limits<u64>::max());
	y.emplace_back(std::numeric_limits<u64>::max());

	isize count = 0, max = 0, min = 0;
	auto xitr = x.cbegin(), yitr = y.cbegin();
	u64 last;
	for (usize i = 0;i != N * 2;++i) {
		if (*xitr < *yitr) {
			++count;
			if (max <= count) {
				max = count;
				last = *yitr;
			}
			++xitr;
		}
		else {
			--count;
			if (count <= min) {
				min = count;
			}
			++yitr;
		}
	}
	const usize diff = max - min;
	if (max == static_cast<isize>(0)) {
		std::abort();
	}
	else if (min == static_cast<isize>(0)) {
		std::cout << y.back() << std::endl;
	}
	else {
		std::cout << L*diff - (L - last) << std::endl;
	}
	return 0;
}