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#include <stdio.h>
#include <time.h>
#include <math.h>
#include <stdlib.h>
#include <string.h>
#include <map> 
#include <string>
#include <vector>  
#include <iostream> 
#include <sstream> 
#include <queue> 
#include <algorithm>

using namespace std;
 
#define ll long long
#define PB 		push_back
#define FOR(a,start,end) 	for(int a=int(start); a<int(end); a++)
#define INF 		INT_MAX
#define SORT(a) 	sort(a.begin(),a.end()) 
#define CL(a,x) 		memset(a,x,sizeof(a))
#define REP(a,x)	for(int a=0;a<x;a++)
#define REP1(a,x)	for(int a=1;a<=x;a++)
#define MP 		make_pair

 
 
typedef vector<int>     vi;
typedef pair<int,int>     pii; 
typedef vector<string> vs;
typedef vector<pii> vii;
typedef vector<vector<int> > vvi;
typedef vector<vector<string> > vvs;
typedef vector<pair<string,string> > vss;
typedef pair<string,string> pss; 
typedef pair<int,pii> piii;
typedef vector<piii> viii;
typedef vector<vector<pii> > vvii;
typedef vector<vvi> vvvi;

ll m,z,i,j,k,n,d,b,c,a;
string w;
ll ddn(ll a){
	ll z;

	z=1;
	z<<=a;
	return z;

}
int main() {   
//	freopen( "c:\\wojtek\\uva\\pa\\debug\\t10.in", "rt", stdin);  
	
	//pi=2*acos(0.0);
//	while(1){
	cin>>n>>k;
	if(n<0)  
		return 0;
	w.clear();
	if(k<=n){
			w+='a';
			while(k>1){
				if(w.back()=='a')
					w+='b';
				else
					w+='a';
			
				k--;
				
			}
			cout<<w<<endl;
			return 0;
	}
	if(n>61){
		d=ddn(62ll);
			w='a';
			
			while(k>1){
				if(k<=(d+1)/2){
					if(w.back()=='a')
						w+='b';
					else 
						w+='a';
					k--;
				}
				else {
					if(w.back()=='c')
						w+='b';
					else 
						w+='c';
					k-=(d+1)/2;
				}
	//			if(n>61)
					n--;
	//			else {
					if(n==61)
						d=ddn(n)-1;
					else if(n<61)
						d/=2;
	//			}
			}
	}
	else {

		d=ddn(n)-1;
	
		if(k>3*d){
			w="NIE";
			cout<<w<<endl;
			return 0;
		}
			
		if(k<=d){
		
			w+='a';
		}
		else if(k<=2*d){
	
			w+='b';
			k-=d;
		}
		else {

			w+="c";
			k-=2*d;
		}
		
	}
	
	while(k>1){
		if(k<=(d+1)/2){
			if(w.back()=='a')
				w+='b';
			else 
				w+='a';
			k--;
		}
		else {
			if(w.back()=='c')
				w+='b';
			else 
				w+='c';
			k-=(d+1)/2;
		}
		d/=2;
	}




	 
	cout<<w<<endl;
//	}
	return 0;

}