This is an exercise number 1 from exam conducted after 1st month of Scala Academy at Adform.
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
Description adapted from: source