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: The string consists of n lowercase English letters (that is, the string's length equals n ), exactly k of these letters are distinct. 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 ≤ i n). Among all strings that meet points 1 and 2, the required string is lexicographically smallest. Help him find such string or state that such string doesn't exist. String x = x 1 x 2... x p is lexicographically less than string y = y 1 y 2... y q , if either p q and x 1 = y 1, x 2 = y 2, ... , x p = y p , or there is such number r (r p, r q), that x 1 = y 1, x 2 = y 2, ... , x r = y r and x r + 1 y r + 1 . The characters of the strings are compared by their ASCII codes.

输入描述:

A single line contains two positive integers n and k(1 ≤ n ≤ 106, 1 ≤ k ≤ 26) — the string's length and the number of distinct letters.

输出描述:

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