#include <iostream>
#include <vector>
#include <cmath>
#include <algorithm>

using namespace std;

int main() {
    ios_base::sync_with_stdio(0);
    cin.tie(0); cout.tie(0);
    int n, m;
    cin >> n >> m;
    vector <int> a(n);
    for (int i = 0; i < n; ++i) {
        cin >> a[i];
    }
    int left = -1, right = m;
    while (right - left > 1) {
        int mid = (right + left) / 2;
        bool ok = true;
        vector <int> b(n);
        for (int i = 0; i < n; ++i) b[i] = a[i];
        if (b[0] == 0 || (b[0] + mid) % m >= 0) {
            b[0] = 0;
        }
        for (int i = 1; i < n; ++i) {
            if (b[i] == b[i - 1]) {
                continue;
            }
            else if (b[i] > b[i - 1]) {
                if (b[i] + mid >= m && (b[i] + mid) % m >= b[i - 1]) {
                    b[i] = b[i - 1];
                }
            }
            else {
                if (b[i - 1] - b[i] > mid) {
                    ok = false;
                    break;
                }
                else {
                    b[i] = b[i - 1];
                }
            }
        }
        if (ok) right = mid;
        else left = mid;
    }
    cout << right << "\n";
    return 0;
}