Given two integers s and d. The task is to find the smallest number such that the sum of its digits is s and the number of digits in the number are d. Return a string that is the smallest possible number. If it is not possible then return -1.

Here is a Python solution for the problem:

def smallest_number(s, d):
    # If sum of digits is 0, then a number is possible
    # only if number of digits is 1. 
    if s == 0:
        if d == 1:
            return "0"
        else:
            return "-1"
 
    # If sum of digits is more than the maximum
    # possible sum 
    if s > 9*d:
        return "-1"
 
    # Create an array to store digits of result
    res = [0]*d
 
    # Fill the first (d-1) digits (from right to left)
    s -= 1
    for i in range(d-1,0,-1):
        # If sum becomes less than 9, then
        # fill the remaining digits by sum
        if s > 9:
            res[i] = 9
            s -= 9
        else:
            res[i] = s
            s = 0
 
    # Fill the last digit with remaining sum
    res[0] = s + 1  # Earlier subtracted 1 to set first digit as 1
 
    # Convert list to string
    smallest_num = ''.join(map(str,res))
 
    return smallest_num

This function works by first checking if the sum of the digits s is 0, in which case it returns "0" if the number of digits d is 1, and "-1" otherwise. Then it checks if s is greater than the maximum possible sum of d digits, returning "-1" if true.

Next, it creates an array res to store the digits of the result. It fills the first d-1 digits (from right to left) with the maximum possible digit (9) until the sum s becomes less than 9, at which point it fills the remaining digits with the remaining sum.

Finally, it fills the last digit with the remaining sum (plus 1, since we subtracted 1 at the beginning to set the first digit as 1), and converts the list of digits to a string to return as the result.