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 ≤ i < n).
3. 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.

小企鹅 Polo 非常喜爱字符串，而他最钟爱长度为 $n$ 的字符串。

某天，他想找出一个满足以下条件的字符串：

1. 该字符串由 $n$ 个小写英文字母组成（即字符串长度为 $n$），其中恰好有 $k$ 个互不相同的字母；
2. 字符串中任意两个相邻字母均不相同；即若将字符串表示为 $s = s_1 s_2 \dots s_n$，则对所有 $1 \le i < n$，均有 $s_i \ne s_{i+1}$；
3. 在所有满足条件 1 和 2 的字符串中，所求字符串是字典序最小的。

请帮助他找到这样的字符串，或判断这样的字符串不存在。

字符串 $x = x_1 x_2 \dots x_p$ 在字典序上小于字符串 $y = y_1 y_2 \dots y_q$，当且仅当满足下列条件之一：

• $p < q$，且 $x_1 = y_1,\, x_2 = y_2,\, \dots,\, x_p = y_p$；
• 或存在某个数 $r$（满足 $r < p$ 且 $r < q$），使得 $x_1 = y_1,\, x_2 = y_2,\, \dots,\, x_r = y_r$，且 $x_{r+1} < y_{r+1}$。
  字符串中字符的比较依据其 ASCII 码值。

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.

一行包含两个正整数 $n$ 和 $k$（$1 ≤ n ≤ 10^6$，$1 ≤ k ≤ 26$）——字符串的长度以及不同字母的个数。

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

在一行中输出所需的字符串。如果不存在这样的字符串，则输出 -1（不带引号）。