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;

class Program
{
    static void Main(string[] args)
    {
        var inputs = Console.ReadLine().Split(' ').Select(txt => int.Parse(txt)).ToList();
        var X = inputs[0];
        var N = inputs[1];

        HashSet<int> candidates = new HashSet<int>();
        if (N == 0)
        {
            candidates.Add(X);
        }
        else
        {
            var Ps = Console.ReadLine().Split(' ').Select(txt => int.Parse(txt)).ToList();
            var pmin = Ps.Min() - 1;
            var pmax = Ps.Max() + 1;
            for (int i = pmin; i <= pmax; i++)
            {
                if (Ps.Contains(i)) continue;
                candidates.Add(i);
            }

            if (X < pmin || pmax < X)
            {
                candidates.Add(X);
            }
        }

        var diff = X;
        var ans = X;
        foreach (var c in candidates.OrderBy(n => n).ToList())
        {
            var d = Math.Abs(c - X);
            if (d < diff)
            {
                diff = d;
                ans = c;
            }
        }

        Console.WriteLine(ans);
    }
}