#include <bits/stdc++.h>
using namespace std;
template <typename T>
ostream &operator<<(ostream &output, const vector<T> &v) {
  if (v.empty()) return output;
  for (int i = 0; i < v.size() - 1; i++) output << v[i] << " ";
  output << v.back();
  return output;
}
template <typename T>
istream &operator>>(istream &input, vector<T> &v) {
  for (auto &i : v) cin >> i;
  return input;
}
void __sol() {
  int n, m;
  cin >> n >> m;
  vector<int> v(n + 1);
  for (int i = 1; i <= n; i++) cin >> v[i];
  int ans = -1;
  auto check = [&](int mid) {
    int mn = 0;
    if (v[1] + mid >= m)
      mn = 0;
    else
      mn = v[1];
    for (int i = 2; i <= n; i++) {
      if (mn > v[i] + mid) return 0;
      if (v[i] > mn) {
        if (v[i] + mid >= m) {
          if ((v[i] + mid) % m < mn) mn = v[i];
        } else
          mn = v[i];
      }
    }
    return 1;
  };
  int l = 0, r = m - 1;
  while (l <= r) {
    int mid = (l + r) / 2;
    if (check(mid))
      ans = mid, r = mid - 1;
    else
      l = mid + 1;
  }
  cout << ans;
  return;
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  int tc = 1;
  while (tc--) __sol();
  return 0;
}
