You are given a string \$s\$ of characters from a to z. Your task is to count how many unique strings of length \$n\$ you can make by concatenating multiple prefixes of the string \$s\$ together.
Since the result can be superlative you can either choose to output the whole result, the result mod \$2^{32}\$ or the result mod \$2^{64}\$.
This is code-golf, shortest code wins.
Tests:
aaaaa 8 -> 1
abcdef 10 -> 492
aerodynamics 10 -> 507
disbelief 10 -> 511