#include <bits/stdc++.h>
using namespace std;
const int MAXN = 300010;
int N, M;
int A[MAXN];
bool check(int v) {
  int prv = 0;
  for (int i = 0; i < N; i++) {
    int t = (prv < A[i] ? prv + M : prv);
    if (A[i] <= t && t <= A[i] + v) {
    } else if (A[i] + v < prv) {
      return false;
    } else {
      prv = A[i];
    }
  }
  return true;
}
int main() {
  ios_base::sync_with_stdio(0), cin.tie(0), cout.tie(0);
  cin >> N >> M;
  for (int i = 0; i < N; i++) {
    cin >> A[i];
  }
  int mi = -1, ma = M;
  while (ma - mi > 1) {
    int md = (mi + ma) / 2;
    if (check(md)) {
      ma = md;
    } else {
      mi = md;
    }
  }
  cout << ma << endl;
  return 0;
}
