using System;
using static System.Console;
using System.Linq;
using System.Collections.Generic;

class Program
{
    static int NN => int.Parse(ReadLine());
    static int[] NList => ReadLine().Split().Select(int.Parse).ToArray();
    public static void Main()
    {
        Solve();
    }
    static void Solve()
    {
        var n = NN;
        var s = NN;
        var bitmax = (int)Math.Pow(3, n);
        for (var b = 0; b < bitmax; ++b)
        {
            var tmp = b;
            var count = new int[3];
            var str = new char[n];
            for (var i = 0; i < n; ++i)
            {
                ++count[tmp % 3];
                str[n - i - 1] = (char)('A' + tmp % 3);
                tmp /= 3;
            }
            if (count[0] > 0 && count[1] > 0 && count[2] > 0) --s;
            if (s == 0)
            {
                WriteLine(string.Concat(str));
                return;
            }
        }
        WriteLine(-1);
    }
}