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 input = INP();
        int x = TOI(input[0]);
        int n = TOI(input[1]);

        if(n==0) {
            CWL(x);
            return;
        }

        var input2 = INP();

        int[] p = new int[n];
        for(int i = 0;i < n;i++) {
            p[i] = TOI(input2[i]);
        }
        Array.Sort(p);

        List<int> rev = new List<int>();
        int current = 1;
        for(int i = 0;i < n;i++) {
            while(true) {
                if(current>p[n-1]) {
                    break;
                }

                if((current) != p[i]) {
                    rev.Add(current);
                } else {
                    current++;
                    break;
                }

                current++;
            }
        }

        int min = int.MaxValue;
        int result = 0;
        foreach(var item in rev) {
            if(min > Math.Abs(item - x)) {
                min = Math.Abs(item - x);
                result = item;
            }
        }

        CWL(result);
    }

    public static List<int> PrimeFactors (int n) {
        var tmpList = new List<int>();
        int i = 2;
        int tmp = n;

        while(i * i <= n) {
            if(tmp % i == 0) {
                tmp /= i;
                tmpList.Add(i);
            } else {
                i++;
            }
        }

        if(tmp != 1) tmpList.Add(tmp);

        return tmpList;
    }

    static string[] INP () {
        return Console.ReadLine().Split(' ');
    }

    static void CWL (string value) {
        Console.WriteLine(value);
    }

    static void CWL (int value) {
        Console.WriteLine(value);
    }

    static void CWL (long value) {
        Console.WriteLine(value);
    }

    static void CWL (float value) {
        Console.WriteLine(value);
    }

    static void CWL (double value) {
        Console.WriteLine(value);
    }

    static int TOI (string str) {
        return int.Parse(str);
    }

    static long TOL (string str) {
        return long.Parse(str);
    }

    static float TOF (string str) {
        return float.Parse(str);
    }

    static double TOD (string str) {
        return double.Parse(str);
    }

    /*static void Main (string[] args) {
        var input = INP();
        var input2 = INP();
        var input3 = INP();

        CWL("dummy");
    }*/
}