#include<bits/stdc++.h>

#define ll long long
#define pi pair<ll,ll>
#define vi vector<ll>
#define cpy(x,y,s) memcpy(x,y,sizeof(x[0])*(s))
#define mset(x,v,s) memset(x,v,sizeof(x[0])*(s))
#define all(x) begin(x),end(x)
#define fi first
#define se second
#define mp make_pair
#define pb push_back
#define ary array
#define eb emplace_back
#define IL inline
#define For(i,j,k) for(ll i=(j);i<=(k);i++)
#define Fol(i,k,j) for(ll i=(k);i>=(j);i--)

using namespace std;

ll read(){
    ll x=0,f=1;char ch=getchar();
    while(ch<'0' || ch>'9')f=(ch=='-'?-1:f),ch=getchar();
    while(ch>='0' && ch<='9')x=(x<<1)+(x<<3)+(ch^48),ch=getchar();
    return x*f;
}
void write(ll x){
    if(x<0)x=-x,putchar('-');
    if(x/10)write(x/10);
    putchar(x%10+'0');
}

void debug(auto &&...x){
    ((cerr<<x<<' '),...);
    cerr<<'\n';
}

vi S,Z;

ll n,m;
ll cnt[31],s[31];
map<vi,ll> t;

bool chk(){
    For(i,0,30)cnt[i]=s[i]=0;
    for(auto v:S)cnt[v]++;
    For(i,1,26){s[cnt[i]]++;if(cnt[i]>n){return false;}}
    Fol(i,n,1){
        s[i]+=s[i+1];
        if(s[i]>(n-i+1))return false;
    }
    //get_id
    Z.clear();
    if(S.size())Z.pb(cnt[S.back()]);
    For(i,1,26)if(cnt[i] && S.size() && S.back()!=i)Z.pb(cnt[i]);
    if(Z.size())sort(Z.begin()+1,Z.end());
    return true;
}
void dfs(){
    if(!chk())return;
    if(t.count(Z) && t[Z]<m){m-=t[Z];return;}
    if(S.size()==n*(n+1)/2){
        if(m==1){for(auto v:S)putchar(v+'a'-1);cout<<endl;exit(0);}
        m--;return;
    }
    ll tmp=m;vi now(Z);
    For(i,1,26){if(!S.size() || S.back()!=i){S.pb(i);dfs();S.pop_back();}}
    return t[now]=(tmp-m),void();
}

int main(){

    //freopen("string.in","r",stdin);
    //freopen("string.out","w",stdout);

    n=read();m=read();dfs();
    write(-1);
    return 0;
}