#include <bits/stdc++.h>
using namespace std;
using ll = long long;
#define rep(i, s, t) for (ll i = s; i < (ll)(t); i++)
#define all(x) begin(x), end(x)

template <typename T> bool chmin(T& x, T y) {
	return x > y ? (x = y, true) : false;
}
template <typename T> bool chmax(T& x, T y) {
	return x < y ? (x = y, true) : false;
}

struct IOST {
	IOST() {
		ios::sync_with_stdio(false);
		cin.tie(nullptr);
		cout << fixed << setprecision(20);
	}
} IOST;

void solve() {
	int n, s;
	cin >> n >> s;
	vector<string> vs;
	auto check = [](const string& t) {
		int flg = 0;
		for (char c : t) flg |= (1 << (c - 'A'));
		return flg == 7;
	};
	{
		vector<string> tmp = {""};
		rep(i, 0, n) {
			vector<string> nx;
			for (auto t : tmp) {
				nx.push_back(t + "A");
				nx.push_back(t + "B");
				nx.push_back(t + "C");
			}
			swap(nx, tmp);
		}
		for (auto t : tmp) {
			if (check(t)) vs.push_back(t);
		}
	}
	sort(vs.begin(), vs.end());
	s--;
	if ((int)vs.size() <= s) cout << "-1\n";
	else cout << vs[s] << "\n";
}

int main() {
	int t = 1;
	// cin >> t;
	rep(i, 0, t) solve();
}