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: Wrong Answer)

use std::collections::HashSet;
use std::io;

fn main() {
    let mut line = String::new();
    io::stdin().read_line(&mut line).unwrap();
    let mut vars = line.split_ascii_whitespace();
    let x = vars.next().unwrap().parse::<u64>().unwrap();
    let n = vars.next().unwrap().parse::<u64>().unwrap();
    assert!(vars.next().is_none());
    line.clear();
    io::stdin().read_line(&mut line).unwrap();
    let ps: HashSet<_> = line
        .split_ascii_whitespace()
        .map(|p| p.parse::<u64>().unwrap())
        .collect();
    assert!(ps.len() == n as usize);
    let ps = (1..=100).filter(|x| !ps.contains(x));
    let mut lt: Option<u64> = None;
    let mut ge: Option<u64> = None;
    for p in ps {
        if p < x {
            if lt.is_none() || lt.unwrap() < p {
                lt = Some(p)
            }
        } else {
            if ge.is_none() || ge.unwrap() > p {
                ge = Some(p)
            }
        }
    }
    if lt.is_none() && ge.is_none() {
        if x > 50 {
            println!("101");
        } else {
            println!("0");
        }
    } else if lt.is_none() {
        println!("{}", ge.unwrap());
    } else if ge.is_none() {
        println!("{}", lt.unwrap());
    } else {
        let lt = lt.unwrap();
        let ge = ge.unwrap();
        if x - lt > ge - x {
            println!("{}", ge);
        } else {
            println!("{}", lt);
        }
    }
}