The super voucher problem with if-statement

The problem * the supermarket is now getting a computer next to the reception to encourage the customers visiting there, there once is a natural numbers n that are 2-digit or 3-digit, 4-digit with no meaningless zeroes.... rule is those customers must enter a number that is at the least, 4-digit and two last digits must equal to sum of two first before number's digits, i.e., computer show out 1311 customers enter 2949 in it. The fastest customer will get a super voucher for a year here.."

My code is:

#include <iostream>
#include <math.h>
using namespace std;
int main() {
    int n;
    cin>>n;
    int x=n;
    int num=log10(n)+1;
    int arr[n]={0};
    while (n) {
        arr[num--]=n%10;
        n/=10;
    }
    if (x>999) {
    
    }
    else if (99<x<=999) {
        arr[4]=arr[3];
        arr[3]=arr[2];
        arr[2]=arr[1];
        arr[1]=0;
    }
    else {
        arr[4]=arr[2];
        arr[3]=arr[1];
        arr[1]=0;
        arr[2]=0;
    }
    if (arr[3]>1)
        cout<<"00";
    else if (arr[3]==1) {
        if (arr[4]==9)
            cout<<"00";
        else
            cout<<arr[3]*10+arr[4]-9<<9;
    }
    else
        cout<<1<<arr[3]*10+arr[4]-1;
    if (arr[1]>1)
        cout<<"00";
    else if (arr[1]==1) {
        if (arr[2]==9)
            cout<<"00";
        else
            cout<<arr[1]*10+arr[2]-9<<9;
    }
    else
        cout<<"0"<<arr[1]*10+arr[2];
}

It's wrong for 1234, output is 0039, but why is that ?? I need to the helps.