#include <bits/stdc++.h>
//#include <atcoder/all>
using namespace std;
//using namespace atcoder;
//using mint = modint1000000007;
//const int mod = 1000000007;
//using mint = modint998244353;
//const int mod = 998244353;
//const int INF = 1e9;
//const long long LINF = 1e18;
#define rep(i, n) for (int i = 0; i < (n); ++i)
#define rep2(i,l,r)for(int i=(l);i<(r);++i)
#define rrep(i, n) for (int i = (n) - 1; i >= 0; --i)
#define rrep2(i,l,r)for(int i=(r) - 1;i>=(l);--i)
#define all(x) (x).begin(),(x).end()
#define allR(x) (x).rbegin(),(x).rend()
#define P pair<int,int>
template<typename A, typename B> inline bool chmax(A & a, const B & b) { if (a < b) { a = b; return true; } return false; }
template<typename A, typename B> inline bool chmin(A & a, const B & b) { if (a > b) { a = b; return true; } return false; }

int main() {
	ios::sync_with_stdio(false);
	cin.tie(nullptr);
	int n, s; cin >> n >> s;
	//s--;
	vector<string>ss;
	auto dfs = [&](auto &&self, vector<int>&v, string &s)->void {
		if (v[0] + v[1] + v[2] == n) {
			if (min({ v[0],v[1],v[2] }) >= 1) {
				ss.push_back(s);
			}
			return;
		}
		rep(i, 3) {
			v[i]++;
			s += 'A' + i;
			self(self,v, s);
			s.pop_back();
			v[i]--;
		}
	};
	vector<int>v(3);
	string now = "";
	dfs(dfs, v, now);
	sort(all(ss));
	if (ss.size() < s) {
		cout << -1 << endl;
	}
	else {
		cout << ss[s - 1] << endl;
	}
	return 0;
}