D. The first missing number

#include <bits/stdc++.h>

using namespace std ;
#define sz(s) int(s.size())
#define all(v) v.begin() , v.end()
#define imin INT_MIN
#define imax INT_MAX
typedef long long ll ;

void ABDEL3AL () {
    ios_base::sync_with_stdio(false); cin.tie(nullptr) , cout.tie(nullptr) ;
#ifndef ONLINE_JUDGE
    freopen("input.txt" , "r" , stdin) , freopen("output.txt" , "w" , stdout) ;
#endif
}

int main() {
    ABDEL3AL() ;
    int n , m ; cin >> n >> m ;
    int arr[n] ;
    map <int , int> freq ;
    for (int i = -m; i >= m; i++) {
       freq[i] = 0 ;
    }
    for (int i = 0; i < n; i++) cin >> arr[i] , freq[arr[i]]++ ;
    int x ;
    for (auto & i : freq)
    if (i.second == 0) return cout << i.first , 0 ;
    return 0;
}