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;
using System.Text;

class Program
{
    static void Main(string[] args)
    {
        var input = Console.ReadLine().Split(" ");
        var X = int.Parse(input[0]);
        var N = int.Parse(input[1]);

        if (N == 0)
        {
            Console.WriteLine(X);
            return;
        }

        var p = new List<int>();
        var line = Console.ReadLine().Split(" ");
        for (int i = 0; i < N; i++)
        {
            p.Add(int.Parse(line[i]));
        };

        var list = new List<int>();
        for (int i = 0; i <= 100; i++)
        {
            list.Add(i);
        }

        foreach(var item in p)
        {
            list.Remove(item);
        }

        var index = ExecuteBinarySearchMin(X, list);
        if(index == list.Count)
        {
            Console.WriteLine(X);
        }
        else if(Math.Abs(list[index - 1] - X) <= Math.Abs(list[index] - X))
        {
            Console.WriteLine(list[index - 1]);
        }
        else
        {
            Console.WriteLine(list[index]);
        }

    }

    // numより数値が小さくなる最大のindexを求める
    static int ExecuteBinarySearchMin(int num, List<int> list)
    {
        int ng = -1; //「index = 0」が条件を満たすこともあるので、初期値は -1
        int ok = list.Count();// 「index = a.size()-1」が条件を満たさないこともあるので、初期値は a.size()

        /* ok と ng のどちらが大きいかわからないことを考慮 */
        while (Math.Abs(ok - ng) > 1)
        {
            int mid = (ok + ng) / 2;

            if (list[mid] > num) ok = mid;
            else ng = mid;
        }
        return ok;
    }
}