#include <bits/stdc++.h>

using namespace std;

typedef long long ll;
typedef pair<ll,ll> P;
#define REP(i,n) for(ll i=0;i<ll(n);i++)








int main(void){
	cin.tie(nullptr);  ios_base::sync_with_stdio(false);
	ll i,j;



	ll N,S;
	cin >> N >> S;

    queue<string> q;
     q.push("A");
     q.push("B");
     q.push("C");

      while(1){

    	  string s=q.front();

    	  if((ll)s.size()==N) break;

          q.pop();

          q.push(s+"A");
          q.push(s+"B");
          q.push(s+"C");

       }


      ll d=0;
      while(!q.empty()){

    	  string s=q.front();
    	  q.pop();

    	  ll a=0,b=0,c=0;
    	  REP(i,N){
    		  if(s[i]=='A') a++;
    		  if(s[i]=='B') b++;
    		  if(s[i]=='C') c++;
    	  }

    	  if(a!=0  && b!=0 && c!=0) d++;

    	  if(d==S){
    		  cout << s << endl;
    		  return 0;
    	  }


      }


      cout << -1 << endl;







	return 0;

}