#include <iostream>
#include <sstream>
#include <iomanip>
#include <vector>
#include <string>
#include <map>
#include <set>
#include <algorithm>
#include <numeric>
#include <iterator>
#include <functional>
#include <complex>
#include <queue>
#include <stack>
#include <cmath>
#include <cassert>
#include <tuple>
using namespace std;
typedef long long LL;
typedef complex<double> CMP;

static const char* BAD = "XXX";

class AB { public:
	map<tuple<int,int,int>, string> memo;
	string create(int N, int KPair, int NumB)
	{
		if(N==0)
			return (KPair==0 && NumB==0 ? "" : BAD);

		tuple<int,int,int> key(N, KPair, NumB);
		if(memo.count(key))
			return memo[key];

		auto a = create(N-1, KPair-NumB, NumB);
		if(a != BAD)
			return memo[key] = "A" + a;
		if(NumB>=1) {
			auto b = create(N-1, KPair, NumB-1);
			if(b != BAD)
				return memo[key] = "B" + b;
		}
		return memo[key] = BAD;
	}

	string createString(int N, int K)
	{
		for(int NumB=0; NumB<=N; ++NumB) {
			auto s = create(N, K, NumB);
			if(s != BAD)
				return s;
		}
		return "";
	}
};

// BEGIN CUT HERE
#include <ctime>
double start_time; string timer()
 { ostringstream os; os << " (" << int((clock()-start_time)/CLOCKS_PER_SEC*1000) << " msec)"; return os.str(); }
template<typename T> ostream& operator<<(ostream& os, const vector<T>& v)
 { os << "{ ";
   for(typename vector<T>::const_iterator it=v.begin(); it!=v.end(); ++it)
   os << '\"' << *it << '\"' << (it+1==v.end() ? "" : ", "); os << " }"; return os; }
void verify_case(const string& Expected, const string& Received) {
 bool ok = (Expected == Received);
 if(ok) cerr << "PASSED" << timer() << endl;  else { cerr << "FAILED" << timer() << endl;
 cerr << "\to: \"" << Expected << '\"' << endl << "\tx: \"" << Received << '\"' << endl; } }
#define CASE(N) {cerr << "Test Case #" << N << "..." << flush; start_time=clock();
#define END	 verify_case(_, AB().createString(N, K));}
int main(){

CASE(0)
	int N = 3; 
	int K = 2; 
	string _ = "ABB"; 
END
CASE(1)
	int N = 2; 
	int K = 0; 
	string _ = "BA"; 
END
CASE(2)
	int N = 5; 
	int K = 8; 
	string _ = ""; 
END
CASE(3)
	int N = 10; 
	int K = 12; 
	string _ = "BAABBABAAB"; 
END
/*
CASE(4)
	int N = ; 
	int K = ; 
	string _ = ; 
END
CASE(5)
	int N = ; 
	int K = ; 
	string _ = ; 
END
*/
}
// END CUT HERE
