package main

import . "fmt"
import . "sort"

func main() {
	var n, s int
	Scan(&n, &s)
	
	t := make([][]string, 8)
	t[0] = append(t[0], "")
	for i := 0; i < n; i++ {
		tmp := make([][]string, 8)
		for i, ts := range t {
			for _, ss := range ts {
				tmp[i|1] = append(tmp[i|1], ss + "A")
				tmp[i|2] = append(tmp[i|2], ss + "B")
				tmp[i|4] = append(tmp[i|4], ss + "C")
			}
		}
		t = tmp
	}
	Strings(t[7])
	
	if s > len(t[7]) {
		Println(-1)
	} else {
		Println(t[7][s-1])
	}
}