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

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

template<int C,class T>
void scan(T &d) {
	int c;
	while (c = fgetc(stdin), c != C) {
		d = d * 10 + c - '0';
	}
}

int main() {
	usize N = 0;
	u64 L = 0;
	scan<' '>(N);
	scan<'\n'>(L);
	std::vector<u64> x(N + 1), y(N + 1);
	const usize li = N - 1;
	for (usize i = 0;i != li;++i) {
		scan<' '>(x[i]);
	}
	scan<'\n'>(x[li]);
	x[N] = L;
	for (usize i = 0;i != li;++i) {
		scan<' '>(y[i]);
	}
	scan<'\n'>(y[li]);
	y[N] = L;

	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;
}