Smallest String With A Given Numeric Value

Given n and k, construct the lexicographically smallest lowercase string of length n whose characters' 1..26 numeric values sum to k (feasible since n ≤ k ≤ 26·n). The core challenge is to distribute the required extra value above the n minimal 'a's across positions so the resulting string is dictionary-minimal, typically by allocating increases as far to the right as possible.