CF288A Polo the Penguin and Strings

Little penguin Polo adores strings. But most of all he adores strings of length $ n $ . One day he wanted to find a string that meets the following conditions: 1. The string consists of $ n $ lowercase English letters (that is, the string's length equals $ n $ ), exactly $ k $ of these letters are distinct. 2. No two neighbouring letters of a string coincide; that is, if we represent a string as $ s=s_{1}s_{2}...\ s_{n} $ , then the following inequality holds, $ s_{i}≠s_{i+1}(1

A single line contains two positive integers $ n $ and $ k $ $ (1

In a single line print the required string. If there isn't such string, print "-1" (without the quotes).