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 Go (1.14.1) (judge verdict: Runtime Error)

package main

import (
	"fmt"
	"os"
	"sort"
)

var x int

type arr []int

func (a arr) Len() int {
	return len(a)
}

func (a arr) Swap(i, k int) {
	a[i], a[k] = a[k], a[i]
}

func (a arr) Less(i, k int) bool {
	d1 := a[i] - x
	d2 := a[k] - x
	if d1 < 0 {
		d1 *= -1
	}
	if d2 < 0 {
		d2 *= -1
	}
	if d1 == d2 {
		return a[i] < a[k]
	}
	if d1 < d2 {
		return true
	}
	return false
}

func main() {
	c, p := input()
	x = c
	sort.Sort(p)
	if len(p) <= 0 {
		fmt.Println(x)
		os.Exit(0)
	}
	if p[0] != x {
		fmt.Println(x)
		os.Exit(0)
	}
	d := 1
	for i := 1; i < len(p); i++ {
		if p[i] != x-d {
			fmt.Println(x - d)
			os.Exit(0)
		}
		i++
		if p[i] != x+d {
			fmt.Println(x + d)
			os.Exit(0)
		}
		d++
	}
	if len(p)%2 == 0 {
		fmt.Println(x - len(p) - 1)
	} else {
		fmt.Println(x + len(p))
	}
}

func input() (int, arr) {
	var x, n int
	fmt.Scan(&x, &n)
	p := make([]int, n, n)
	for i := range p {
		fmt.Scan(&p[i])
	}
	return x, p
}