vecine

/*
Implementare Dan Pracsiu - 100p
*/
#include <iostream>
#include <fstream>
using namespace std;

ifstream  fin("vecine.in");
ofstream fout("vecine.out");
int n, a[100003];

void F1()
{
    int i, cnt = 0;
    for (i = 1; i < n; i++)
        if (a[i] + 1 == a[i + 1]) cnt++;
    fout << cnt << "\n";
}

void F2()
{
    int i, j, L, k;
    long long x, y, ans = -1;
    L = min(10, n / 2);
    for (k = L; k >= 1; k--)
    {
        /// cautam daca exista doua numere alaturate consecutive de k cifre
        for (i = 1; i <= n - 2 * k + 1; i++)
            if (a[i] != 0)
            {
                x = y = 0;
                for (j = 0; j < k; j++)
                    x = x * 10 + a[i + j];
                for (j = 0; j < k; j++)
                    y = y * 10 + a[i + k + j];
                if (x + 1 == y)
                    ans = max(ans, x);
                /// verific daca x si y nu sunt de forma x=999, y=1000
                if (i + 2 * k <= n)
                {
                    y = y * 10 + a[i + 2 * k];
                    if (x + 1 == y) ans = max(ans, x);
                }
            }
        else if (k == 1 && a[i + 1] == 1)
                    ans = max(ans, 0LL);
    }
    fout << ans << "\n";
}

int main()
{
    int task;
    fin >> task >> n;
    for (int i = 1; i <= n; i++)
        fin >> a[i];
    if (task == 1) F1();
    else F2();
    fin.close();
    fout.close();
    return 0;
}