#include <bits/stdc++.h>   using namespace std;   const int MXX = 3e5 + 5;   int n, m;   int a[MXX], ans[MXX];   bool ok(int x) {     int mx = m - 1;     for (int i = n - 1; i >= 0; i--) {       if (a[i] <= mx) {         if (a[i] + x >= mx) {           ans[i] = mx;         } else {           ans[i] = a[i] + x;         }       } else {         if (a[i] + x >= mx + m)           ans[i] = mx;         else           ans[i] = (a[i] + x) % m;       }       mx = ans[i];     }     for (int i = 0; i < n - 1; i++) {       if (ans[i] > ans[i + 1]) {         return false;       }     }     return true;   }   bool ok2() {     for (int i = 0; i < n - 1; i++) {       if (a[i] > a[i + 1]) {         return false;       }     }     return true;   }   int main() {     cin >> n >> m;     for (int i = 0; i < n; i++) {       cin >> a[i];     }     if (ok2()) {       return cout << 0 << endl, 0;     }     int lo = 0, hi = m - 1, mid, ans = -1;     while (lo <= hi) {       mid = (lo + hi) / 2;       if (ok(mid)) {         ans = mid;         hi = mid - 1;       } else {         lo = mid + 1;       }     }     cout << ans << endl;     return 0;   }   