648_DIV_2_1000.cpp

#include <iostream>
#include <string>
#include <cstring>
#include <vector>
#include <cmath>

using namespace std;

int n,k;
vector<char> str(35);
bool bl[32][32][32][450];

bool dp(int i, int a, int b, int q){
        if(i == n){
                if(q == k) return 1;
                return 0;
        }
        if(bl[i][a][b][q] == 1) return 0;
        bl[i][a][b][q] = 1;
        str[i] = 'A';
        if(dp(i+1, a+1, b, q) == 1) return 1;
        str[i] = 'B';
        if(dp(i+1, a, b+1, q+a) == 1) return 1;
        str[i] = 'C';
        if(dp(i+1, a, b, q+a+b) == 1) return 1;
        return 0;
}

int main(){
        cin >> n >> k;
        if(dp(0,0,0,0) == 0)
                cout << "Empty string" << endl;
        else {
                string rs = "";
                for(int i=0;i<n;i++) rs = rs + str[i];
                cout << rs << endl;
        }
        return 0;
}