Challenge Assign every lowercase letter a value, from 1 for a to 26 for z. Given a string of lowercase letters, find the sum of the values of the letters in the string.

lettersum("") => 0

lettersum("a") => 1

lettersum("z") => 26

lettersum("cab") => 6

lettersum("excellent") => 100

lettersum("microspectrophotometries") => 317