openol A 100

#include <bits/stdc++.h>
// tiom4eg's precompiler options
// POGGERS POGGERS POGGERS POGGERS POGGERS POGGERS POGGERS
// IO settings
#define fastIO ios_base::sync_with_stdio(false); cin.tie(0)
// Quick types
#define ll long long
#define ld long double
//#define ull unsigned long long
#define pii pair <int, int>
#define vi vector <int>
#define mi vector <vector <int>>
// Quick functions
#define endl "\n"
#define F first
#define S second
#define all(a) a.begin(), a.end()
#define sz(a) (int)(a.size())
#define pb push_back
#define mp make_pair
// Quick fors
#define FOR(i, a, b) for (int i = a; i < b; ++i)
#define FORS(i, a, b, c) for (int i = a; i < b; i += c)
#define RFOR(i, a, b) for (int i = a; i >= b; --i)
#define EACH(e, a) for (auto& e : a)
// Pragmas
#ifndef TIOM4EG
#pragma GCC optimize("O3") // let the chaos begin!
#pragma GCC target("avx,avx2,tune=native")
#pragma GCC comment(linker, "/stack:200000000")
#endif
// PBDS
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
#define pbds tree <int, null_type, less <int>, rb_tree_tag, tree_order_statistics_node_update>
using namespace __gnu_pbds;
// POGGERS POGGERS POGGERS POGGERS POGGERS POGGERS POGGERS
using namespace std;
mt19937 rng(chrono::duration_cast<chrono::milliseconds>(chrono::system_clock::now().time_since_epoch()).count());
//#define int long long
constexpr int INF = 1e9 + 7, MD = 998244353, MAX = 200007, LG = 19, R = 1 << LG, MOD = 1000000007, MOD2 = 1e9 + 9, B = 256;
const ll INFLL = 1e18 + 7;

int t[2 * R];
void upd(int l, int r) {
    for (l += R, r += R; l < r; l >>= 1, r >>= 1) {
        if (l & 1) t[l++]++;
        if (~r & 1) t[r--]++;
    }
    if (l == r) t[l]++;
}
int get(int p) {
    int s = 0;
    for (p += R; p; s += t[p], p >>= 1) {}
    return s;
}

signed main() {
    fastIO;
    int n, m; cin >> n >> m;
    vi a(n); EACH(e, a) cin >> e;
    vector <pii> qs;
    FOR(i, 0, n - 1) if (a[i + 1] - a[i] > m) {
        if (a[i + 1] - a[i] >= 2 * m) return !(cout << -1);
        int l = (a[i] + 1) % m, r = (a[i + 1] - m + 1) % m;
        if (l < r) qs.pb({l, r});
        else {
            qs.pb({l, m});
            if (r) qs.pb({0, r});
        }
    }
    vi xs;
    xs.pb(0), xs.pb(m);
    EACH(e, qs) xs.pb(e.F), xs.pb(e.S);
    sort(all(xs)), xs.erase(unique(all(xs)), xs.end());
    EACH(e, qs) upd(lower_bound(all(xs), e.F) - xs.begin(), lower_bound(all(xs), e.S) - (xs.begin() + 1));
    FOR(i, 0, sz(xs) - 1) if (!get(i)) return !(cout << xs[i]);
    cout << -1;
}