#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
#define int ll
#define rep(i,a,b) for(int i = a; i < (b); i++)
#define all(x) begin(x), end(x)
#define sz(x) (int)(x).size()
#define PB push_back
#define FS first
#define SD second
#define cmx(x, y) x = max(x, y)
#define cmn(x, y) x = min(x, y)
#define ary(k) array<int, k>
typedef pair<int, int> pii;
typedef vector<int> vi;
map <vector<int>,int> mp;
long long inf=1000000000000000001;
int k;
long long solve(vector<int> v){
    sort(v.begin()+1,v.end());
    if(mp.count(v)) return mp[v];
    int f[27]={};
    for(int i=0;i<v.size();i++) {
        if(v[i]>k) return mp[v]=0;
        for (int j = 1; j <= v[i]; j++)
            f[j]++;
    }
    int gd=1;
    for(int i=k;i;i--){
        if(f[i]>k-i+1) return mp[v]=0;
        if(f[i]!=k-i+1) gd=0;
    }
    //cout<<"#";for(auto t:v) cout<<t<<" ";cout<<"\n";
    //cout<<gd<<'\n';
    if(gd) return mp[v]=1;
    long long ans=0;
    for(int i=1;i<v.size();i++){
        vector <int> nxt;
        nxt.push_back(v[i]+1);
        for(int j=0;j<v.size();j++) if(j!=i) nxt.push_back(v[j]);
        int temp=solve(nxt);
        ans+=temp;
        if(ans>=inf) return mp[v]=inf;
    }
    if(v.size()<k) {
        vector <int> nxt;
        nxt.push_back(1);
        for (int i = 0; i < v.size(); i++) nxt.push_back(v[i]);
        //cout<<"?";for(auto t:nxt) cout<<t<<" ";cout<<"\n";
        int temp = solve(nxt);
        if(temp) {
            if (inf / temp <= 26 - v.size()) return mp[v] = inf;
            ans += temp * (26 - v.size());
            if (ans > inf) return mp[v] = inf;
        }
    }
    //for(auto t:v) cout<<t<<" ";cout<<ans<<'\n';
    return mp[v]=ans;
}
signed main() {
	cin.tie(0)->sync_with_stdio(0); cin.exceptions(cin.failbit);
    int n;
    cin>>k>>n;
    char lst='?';
    int rest=n;
    vector <int> v;
    int f[27]={};
    for(int i=1;i<=k*(k+1)/2;i++){
        int cnt=0;
        for(int j=0;j<26;j++)
            if(j+'a'!=lst){
                vector <int> nxt;
                if(!f[j]){
                    nxt.push_back(1);
                    for(auto t:v) nxt.push_back(t);
                    int oc=cnt;
                    cnt+=solve(nxt);
                    if(cnt>=rest){
                        rest-=oc;
                        v=nxt;
                        sort(v.begin()+1,v.end());
                        f[j]++,lst=j+'a';
                        cout<<lst;
                        break;
                    }
                }
                else{
                    int pos=-1;
                    for(int l=1;l<v.size();l++) if(v[l]==f[j]){
                        pos=l;
                        break;
                    }
                    if(pos!=-1){
                        vector <int> nxt;
                        nxt.push_back(v[pos]+1);
                        for(int l=0;l<v.size();l++)
                            if(l!=pos)
                                nxt.push_back(v[l]);
                        int oc=cnt;
                        cnt+=solve(nxt);
                        if(cnt>=rest){
                            rest-=oc;
                            v=nxt;
                            sort(v.begin()+1,v.end());
                            f[j]++,lst=j+'a';
                            cout<<lst;
                            break;
                        }
                    }
                }
                //cout<<j<<" "<<cnt<<'\n';
            }
        if(cnt<rest){
            cout<<"-1\n";
            return 0;
        }
    }
    cout<<'\n';
}