Missing_Number.cpp

#include <bits/stdc++.h>
    using namespace std;
    #define FasterIO ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0);
    #define int long long
    #define FOR(i,n) for(int i=0;i<n;i++)
    #define a_sort(v) sort(v.begin(),v.end())
    #define d_sort(v) sort(v.begin(),v.end(),greater<int>())
    #define YES cout << "YES" << endl;
    #define NO cout << "NO" << endl;
    #define vi vector<int>
    #define vl vector<long long>
    #define maximum(v) *max_element(v.begin(),v.end())
    #define minimum(v) *min_element(v.begin(),v.end())
    #define unq(v) v.resize(distance(v.begin(),unique(v.begin(),v.end())))
     
    void solve(){
       int n;
       cin >> n;
       vi a(n),b(n-1);
       FOR(i,n){
        a[i]=i+1;
       }
       FOR(i,n){
        cin >> b[i];
       }
       a_sort(b);
       for(int i=0;i<n;i++){
            if(a[i]!=b[i]){
                
                cout << a[i] << '\n';
                return;
            }
       }
    //cout << b[] << '\n';
    }
     
    
    int32_t main(){
        FasterIO;
        solve();
     
    }