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 Rust (1.42.0) (judge verdict: Accepted)

#![allow(unused_imports)]

//input
use proconio::input;
use proconio::marker::{Bytes, Chars};
//use std::io::Read;

//collections
//use std::collections::BTreeMap;
//use std::collections::BTreeSet;
//use std::collections::HashMap;
use std::collections::HashSet;
//use std::collections::VecDeque;
//use std::collections::BinaryHeap;

//use std::cmp::*;

fn main() {
    input! {
        x: i64,
        n: usize,
    }

    if n == 0 {
        println!("{}", x);
        return;
    }

    input! {
        p:[i64;n]
    }

    let dic: HashSet<i64> = p.into_iter().collect();

    for i in 0.. {
        if !dic.contains(&(x - i)) {
            println!("{}", x - i);
            return;
        }
        if !dic.contains(&(x + i)) {
            println!("{}", x + i);
            return;
        }
    }
}