#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
const long double PI =
    3.141592653589793238462643383279502884197169399375105820974944;
inline void io() {}
const long long mod = 1e18 + 7;
const long long maxn = 3e5 + 5;
long long n, m, a[maxn];
vector<long long> v;
bool go(long long x) {
  long long mx = 0;
  bool ok = false;
  for (long long i = 0; i < n; i++) {
    if (x + a[i] < mx) return ok;
    if (a[i] <= mx) continue;
    if (a[i] + x >= m && (a[i] + x) % m >= mx) continue;
    mx = a[i];
  }
  ok = !ok;
  return ok;
}
void solve() {
  cin >> n >> m;
  for (long long i = 0; i < n; i++) cin >> a[i];
  long long lb = 0, rb = m;
  while (lb < rb - 1) {
    long long mid = (lb + rb) / 2;
    if (go(mid)) {
      rb = mid;
    } else {
      lb = mid;
    }
  }
  if (go(lb)) {
    cout << lb;
  } else
    cout << rb;
}
signed main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  long long tc = 1;
  while (tc--) {
    solve();
  }
  return 0;
}
