How to find out missing value in the array in cpp?

Here is my solution to given problem in cpp:

#include <iostream>
 using namespace std;
 int main()
{
         int a[20],i,n,t,j,k;
    cin>>t;
              for(i = 0;i<t;i++){
        cin>>n;
                 int arr[20] = {};
        for(k = 0;k<n-1;k++){
            cin>>a[k];
        }
                 for(j=0;j<n-2;j++){
            if((a[j+1]-a[j])!=1)
            {
                cout<<a[j]+1<<"\n";
            }
        }
             }
     return 0;
}

output:

4
9