This time no story, no theory. The examples below show you how to write function accum:
Examples:
accum("abcd"); // "A-Bb-Ccc-Dddd"
accum("RqaEzty"); // "R-Qq-Aaa-Eeee-Zzzzz-Tttttt-Yyyyyyy"
accum("cwAt"); // "C-Ww-Aaa-Tttt"
The parameter of accum is a string which includes only letters from a..z and A..Z.