#include <iostream>
#include <vector>
#include <algorithm>
using namespace std;
using ll = long long;

int main() {
  ll n, m;
  cin >> n >> m;
  vector<ll> a(n), b(n);
  for (ll i = 0; i < n; i++)
    cin >> a[i];
  for (ll i = 0; i < n; i++) {
    cin >> b[i];
    b[i] += m - a[i];
    b[i] %= m;
  }
  b.insert(b.begin(), 0);
  b.push_back(0);
  vector<ll> db(n + 1);
  ll plus = 0;
  for (ll i = 0; i < n + 1; i++) {
    db[i] = (m + b[i + 1] - b[i]) % m;
    plus += b[i + 1] >= b[i];
  }
  ll ans = 0;
  sort(db.begin(), db.end());
  for (ll i = 0; i < plus; i++) {
    ans += db[i];
  }
  cout << ans << endl;
  return 0;
}