// #include <bits/allocator.h> // Temp fix for gcc13 global pragma
// #pragma GCC target("avx2,bmi2,popcnt,lzcnt")
// #pragma GCC optimize("O3,unroll-loops")
#include <bits/stdc++.h>
// #include <x86intrin.h>
using namespace std;
#if __cplusplus >= 202002L
using namespace numbers;
#endif
#ifdef LOCAL
	#include "Debug.h"
#else
	#define debug_endl() 42
	#define debug(...) 42
	#define debug2(...) 42
	#define debug_bin(...) 42
#endif



int main(){
	cin.tie(0)->sync_with_stdio(0);
	cin.exceptions(ios::badbit | ios::failbit);
	int n, k;
	cin >> n >> k, -- k;
	vector<int> a(n);
	a[n - 2] = 1, a[n - 1] = 2;
	while(k --){
		while(true){
			++ a[n - 1];
			for(auto i = n - 1; a[i] == 3; -- i){
				if(i == 0){
					cout << "-1\n";
					return 0;
				}
				a[i] = 0;
				++ a[i - 1];
			}
			array<bool, 3> flag{};
			for(auto c: a){
				flag[c] = true;
			}
			if(ranges::min(flag)){
				break;
			}
		}
	}
	for(auto c: a){
		cout << "ABC"[c];
	}
	cout << "\n";
	return 0;
}

/*

*/