Problem:
Score : 300 points

Problem Statement

Given are an integer X and an integer sequence of length N: p_1, \ldots, p_N.

Among the integers not contained in the sequence p_1, \ldots, p_N (not necessarily positive), find the integer nearest to X, that is, find the integer whose absolute difference with X is the minimum. If there are multiple such integers, report the smallest such integer.

Constraints

1 \leq X \leq 100

0 \leq N \leq 100

1 \leq p_i \leq 100

p_1, \ldots, p_N are all distinct.

All values in input are integers.

Input

Input is given from Standard Input in the following format:

X N
p_1 ... p_N

Output

Print the answer.

Sample Input 1

6 5
4 7 10 6 5

Sample Output 1

8

Among the integers not contained in the sequence 4, 7, 10, 6, 5, the one nearest to 6 is 8.

Sample Input 2

10 5
4 7 10 6 5

Sample Output 2

9

Among the integers not contained in the sequence 4, 7, 10, 6, 5, the ones nearest to 10 are 9 and 11. We should print the smaller one, 9.

Sample Input 3

100 0

Sample Output 3

100

When N = 0, the second line in the input will be empty. Also, as seen here, X itself can be the answer.

Solution in C# (.NET Core 3.1.201) (judge verdict: Wrong Answer)

using System;
using System.Collections.Generic;
using System.Linq;

namespace _170_c
{
    internal class ForbiddenList
    {
        private static void Main(string[] args)
        {
            var xn = Console.ReadLine()?.Split(" ").Select(int.Parse).ToArray();
            if (xn == null) return;
            var x = xn[0];
            var n = xn[1];

            if (n == 0)
            {
                Console.ReadLine();
                Console.WriteLine(x);
                return;
            }

            var p = Console.ReadLine()?.Split(" ").Select(int.Parse).ToList();
            if (p == null) return;

            if (!p.Contains(x))
            {
                Console.WriteLine(x);
                return;
            }

            p.Add(x);
            p.Sort();

            var index = p.IndexOf(x);

            var largerP = (index == p.Count) ? new List<int> {p[p.Count]} : p.GetRange(index + 1, n - index);
            var smallerP = (index == 0) ? new List<int> {p[0]} : p.GetRange(0, index);
            smallerP.Reverse();

            var lCount = 0;
            var sCount = 0;

            for (; lCount < largerP.Count; lCount++)
            {
                var diff = largerP[lCount] - x;
                if (diff > lCount + 1) break;
            }

            for (; sCount < smallerP.Count; sCount++)
            {
                var diff = x - smallerP[sCount];
                if (diff > sCount + 1) break;
            }

            if (sCount == 0)
            {
                Console.WriteLine(x - 1);
                return;
            }

            if (lCount == 0)
            {
                Console.WriteLine(x + 1);
                return;
            }

            if (x - smallerP[sCount - 1] <= largerP[lCount - 1] - x)
            {
                Console.WriteLine(smallerP[sCount - 1] - 1);
            }
            else
            {
                Console.WriteLine(largerP[lCount - 1] + 1);
            }
        }
    }
}