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

using isize = int;
using usize = unsigned int;
using u64 = unsigned long long;

int main() {
	usize N;
	u64 L;
	scanf("%u%llu", &N, &L);
	std::vector<u64> x(N);
	for (auto &e : x) {
		scanf("%llu", &e);
	}
	std::vector<u64> y(N);
	for (auto &e : y) {
		scanf("%llu", &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;
	u64 ans;
	if (max == static_cast<isize>(0)) {
		ans = L*diff + y.front();
	}
	else {
		ans = L*diff - (L - last);
	}
	printf("%llu\n", ans);
	return 0;
}