The task of displaying numbers - c++

Question

Can't complete the task and need your help. With single digits all OK, but when i trying numbers like 9865, output is wrong.
Task:
Input:
The first line contains an integer 𝑛 (0 ≤ 𝑛 ≤ 1e6). The second line contains nine separated by a space
integers 𝑎1, 𝑎2,…, 𝑎9(1 ≤ 𝑎𝑖 ≤ 1e5) - the value in gold of each digit (from 1 to 9, respectively).
Output:
Print the maximum number for which the player has enough gold. The value of the number is equal to the sum of the values
each digit included in it. If the player does not have enough gold for any number, then output −1
EXAMPLE:
Input:
n = 2
a[9] = {1 2 3 4 4 4 4 4 4}
Output:
11
Note:
In the example, two units of gold may only be enough for one digit 2 or two digits 1, so the maximum number that can be applied is 11.
#include <iostream>
#include <string>
#include <algorithm>
using namespace std;
const int MAX = 9;
int main()
{
int a[MAX]; // array
int b[MAX];
unsigned int n; // gold
cin >> n;
for (int i = 0; i < MAX; i++)
{
cin >> a[i];
}
for (int i = 0; i < MAX; i++)
{
int count = n / a[i];
int post = n % a[i];
if (count == 0)
{
b[i] = 0;
}
else
{
for (int k = 0; k < count; k++)
{
if (post == 0)
{
string prom = to_string(a[i]); // int to string
if (count > 1)
{
prom += to_string(a[i]);
}
b[i] = atoi(prom.c_str()); // string to int
}
else
{
string prom = to_string(a[i]); // int to string
if (count > 1)
{
prom += to_string(a[i]) + to_string(post);
}
b[i] = atoi(prom.c_str()); // string to int
}
}
}
}
int maxnum = 0;
for (int i = 0; i < MAX; i++)
{
if (b[i] > maxnum)
{
maxnum = b[i];
}
}
cout << maxnum;
return 0;
}