current to 0 to hold the value of the current substring being formed.count to 1 as we will have at least one substring.d in the string s.
a. Convert d to an integer.
b. If d is greater than k, return -1 immediately.
c. Multiply current by 10 and add d to current.
d. If current is now greater than k, increment count, and set current to d.count as the minimum number of substrings.