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#include<string>
#include<stdio.h>
#include<iostream>
#include<set>
#include<vector>
#include<algorithm>
#include<queue>
#include<bitset>
#define ll long long
#define pb push_back
#define sc scanf
#define pr printf
#define f first
#define s second
#define mp make_pair
#define lb lower_bound

using namespace std;

char t[1000000];

void ws(int x,char c){
    if(t[x-1]<=c)c++;
    t[x]=c;
    return;
}

int main (){
    int n;
    ll k;
    scanf("%d%lld",&n,&k);
    ll p;
    if(n<=60){
        p=(1<<n)-1;
        if(k<=p){
            k-=1;
            t[0]='a';
        }
        else if(k<=p+p){
            k-=p+1;
            t[0]='b';
        }
        else if(k<=p+p+p){
            k-=p+p+1;
            t[0]='c';
        }
        else{
            printf("NIE");
            return 0;
        }
    }
    else{
        k-=1;
        t[0]='a';
    }
    n--;
    int x=1;
    //printf("tu");
    while(n>0){
        if(k<=0)break;
        if(n>=60)p=((ll)(1))<<60;
        else p=((ll)(1))<<n;
        if(k>=p){
            k-=p;
            ws(x,'b');
            x++;
        }
        else{
            ws(x,'a');
            k-=1;
            x++;
        }
        n--;
    }
    for(int i=0;i<x;i++)printf("%c",t[i]);
    return 0;
}