#include <bits/stdc++.h>
using namespace std;
const int mod = 1000000003, MAX = 100003, INF = 1 << 30;
int main() {
  int N, M;
  cin >> N >> M;
  vector<int> A(N);
  for (int i = 0; i < N; i++) cin >> A[i];
  bool ok = true;
  for (int i = 1; i < N; i++)
    if (A[i - 1] > A[i]) ok = false;
  if (ok) {
    cout << 0 << endl;
    return 0;
  }
  int left = 0, right = M;
  while (right - left > 1) {
    int mid = (left + right) / 2;
    bool ok = true;
    int now;
    if (A[0] + mid >= M)
      now = 0;
    else
      now = A[0];
    for (int i = 1; i < N; i++) {
      if (now == A[i]) continue;
      if (now > A[i]) {
        if (A[i] + mid >= now)
          continue;
        else
          ok = false;
      } else {
        if (A[i] + mid < M)
          now = A[i];
        else {
          if ((A[i] + mid) % M >= now)
            continue;
          else
            now = A[i];
        }
      }
    }
    if (ok)
      right = mid;
    else
      left = mid;
  }
  cout << right << endl;
}
