Input (grade 5): Each unit (1, 2,..., 9) is assigned a separate letter, each tens (10, 20,..., 90) a separate letter, and the first four hundreds (100, 200, 300, 400) a separate letter.
Output (grade 4): Each unit (1, 2,..., 9) is assigned a separate letter, each tens (10, 20,..., 90) a separate letter, and each hundreds (100, 200,..., 900) a separate letter.