|Codeforces Round #177 (Div. 1)|
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:
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.
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.
In a single line print the required string. If there isn't such string, print "-1" (without the quotes).