#include <bits/stdc++.h>
using namespace std;
using ll = long long;
int main () {
	int N; ll M;
	cin >> N >> M;
	std::vector<ll> A(N), B(N);
	for (auto& a : A) {
		cin >> a;
	}
	for (auto& a : B) {
		cin >> a;
	}
	for (int i = 0; i < N; i ++) {
		A[i] = (B[i] - A[i] + M) % M;
	}
	vector<ll> C(N + 1);
	C[0] = A[0];
	for (int i = 1; i < N; i ++) {
		C[i] = (A[i] - A[i - 1] + M) % M;
	}
	sort(C.begin(), C.end());
	ll aft = M * (N + 1) - accumulate(C.begin(), C.end(), 0ll);
	ll bef = 0;
	ll ans = aft;
	for (int i = 0; i <= N; i ++) {
		bef += C[i];
		aft -= (M - C[i]) % M;
		ans = min(ans, max(bef, aft));
	}
	cout << ans << endl;
}