You’re given two strings, source and target, made up of lowercase English letters. Your task is to determine the minimum number of characters that must be appended to the end of the source so that the target becomes a subsequence of the resulting string.

Note: A subsequence is formed by deleting zero or more characters from a string without changing the order of the remaining characters.

Constraints:

• $1 \leq$ source.length, target.length $\leq 10^3$ ...