#include <bits/stdc++.h>
using namespace std;

#define rep(i, a, b) for (ll i = a; i < b; i++)
using ll = long long;

int main(){
    int n, s;
    cin >> n >> s;
    int d = 0;
    vector<int> v;
    while(1) {
        vector<int> c(3);
        int x = d;
        int k = n;
        while(x) {
            int m = x % 3;
            c[m]++;
            x /= 3;
            k--;
        }
        if(k < 0) break;
        if((c[1] && c[2] && k > 0) || (c[0] && c[1] && c[2])) {
            v.push_back(d);
        }
        d++;
    }
    int sz = v.size();
    if(sz < s) {
        cout << -1 << endl;
        return 0;
    }
    int x = v[s - 1];
    string t;
    int k = n;
    while(x) {
        int m = x % 3;
        t += char(m + 'A');
        x /= 3;
        k--;
    }
    rep(i, 0, k) t += 'A';
    reverse(t.begin(), t.end());
    cout << t << endl;
}