Next smaller number with the same digits in C language - 4 kyu

Next smaller number with the same digits in C language - 4 kyu

/*Doesn't work on 970*/
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <assert.h>

unsigned long long int next_smaller_number(unsigned long long int number);
int main(int argc, char const *argv[])
{
    unsigned long long int result = 0;
    result = next_smaller_number(441);
    assert(result == 414);
    printf("Test Passed\n");
    return 0;
}

unsigned long long int next_smaller_number(unsigned long long int number)
{
    if (number < 10)
    {
        return -1;
    }

    unsigned long long int result = 0;
    char *numberString = malloc(sizeof(char) * 30);
    sprintf(numberString, "%llu", number);
    int length = strlen(numberString), position = length - 1;
    for (int i = length - 1; i > 0; i--)
    {

        if (numberString[i] < numberString[i - 1])
        {

            position = (i - 1);
            break;
        }
    }

    char temp = numberString[position];
    numberString[position] = numberString[position + 1];
    numberString[position + 1] = temp;

    for (int i = position + 1; i < length + 2; i++)
    {
        if (numberString[i] < numberString[i + 1])
        {
            char temp = numberString[i];
            numberString[i] = numberString[i + 1];
            numberString[i + 1] = temp;
        }
    }

    if (numberString[0] == '0')
    {
        return -1;
    }

    result = atoll(numberString);
    return result;
}