/*
 *                                                     __----~~~~~~~~~~~------___
 *                                    .  .   ~~//====......          __--~ ~~
 *                    -.            \_|//     |||\\  ~~~~~~::::... /~
 *                 ___-==_       _-~o~  \/    |||  \\            _/~~-
 *         __---~~~.==~||\=_    -_--~/_-~|-   |\\   \\        _/~
 *     _-~~     .=~    |  \\-_    '-~7  /-   /  ||    \      /
 *   .~       .~       |   \\ -_    /  /-   /   ||      \   /
 *  /  ____  /         |     \\ ~-_/  /|- _/   .||       \ /
 *  |~~    ~~|--~~~~--_ \     ~==-/   | \~--===~~        .\
 *           '         ~-|      /|    |-~\~~       __--~~
 *                       |-~~-_/ |    |   ~\_   _-~            /\
 *                            /  \     \__   \/~                \__
 *                        _--~ _/ | .-~~____--~-/                  ~~==.
 *                       ((->/~   '.|||' -_|    ~~-/ ,              . _||
 *                                  -_     ~\      ~~---l__i__i__i--~~_/
 *                                  _-~-__   ~)  \--______________--~~
 *                                //.-~~~-~_--~- |-------~~~~~~~~
 *                                       //.-~~~--\
 *                       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 *
 *                               神兽保佑            永无BUG
 */

/*
 * @Description: I want to be the weakest vegetable in the world!
 * @Author: I.Z.
*/
#include<bits/stdc++.h>
using namespace std;
#define MOD         998244353
#define speMOD      2933256077ll
#define int         long long
#define pii         pair<int,int>
#define all(v)      v.begin(),v.end()
#define pb          push_back
#define REP(i,b,e)  for(int i=(b);i<(int)(e);++i)
#define over(x)     {cout<<(x)<<endl;return;}
#define lowbit(x)   ((x)&(-(x)))
#define cntbit(x)   __builtin_popcount(x)
#define deal(v)     sort(all(v));v.erase(unique(v.begin(),v.end()),v.end())
#define lbound(v,x) lower_bound(all(v),x)-v.begin()
mt19937 sd(random_device{}());
int qpow(int a,int b,int m=MOD,int res=1){
	a%=m;
	while(b>0)res=(b&1)?(res*a%m):(res),a=a*a%m,b>>=1;
	return res;
}
int exgcd(int x,int y,int &a,int &b){
	if(y==0){
		a=1;b=0;
		return x;
	}
	int d=exgcd(y,x%y,a,b);
	int z=b;
	b=a-b*(x/y);
	a=z;
	return d;
}
int _x_,_y_;
inline int inverse(int x,int y=MOD){
	exgcd(x,y,_x_,_y_);
	return (_x_+y)%y;
}
int fac[2000005],inv[2000005];
void init(int n){
	fac[0]=inv[0]=1;
	REP(i,1,n+1)fac[i]=fac[i-1]*i%MOD;
	inv[n]=qpow(fac[n],MOD-2);
	for(int i=n-1;i>=1;--i)inv[i]=inv[i+1]*(i+1)%MOD;
}
int C[700][700];
int binom(int x,int y){
	return x<y||y<0? 0:C[x][y];
}
int inf=1000000000000000001;
int mul(int x,int y){
    return (int)(min(__int128(inf),x*(__int128)(y)));
}
struct solver{
	int n;
	int a[26];
	int f[30][360];
	int solve(vector<int>b,int id){
		n=b.size();
		REP(i,0,n)a[i]=b[i];
		swap(a[id],a[0]);
		sort(a+1,a+n,greater<int>());
		while(n>1&&!a[n-1])--n;
		if(n>1)sort(a+1,a+n);
		int sum=accumulate(a,a+n,0ll);
		REP(i,0,n+1)REP(j,0,sum+1)f[i][j]=0;
		f[1][a[0]-1]=1;
		int s=a[0];
		REP(i,1,n){
			REP(l,0,s+1)if(f[i][l]){
				REP(j,1,a[i]+1)if(j<=s){
					REP(k,0,l+1)if(k<=j){
						int w=l-k+a[i]-j;
                        f[i+1][w]=min(inf,f[i+1][w]+mul(mul(f[i][l],binom(l,k)),mul(binom(s-l,j-k),binom(a[i]-1,j-1))));
					}else break;
				}else break;
			}
			s+=a[i];
		}
		return f[n][0];
	}
	int solve(vector<int>b){
		n=b.size();
		REP(i,0,n)a[i]=b[i];
		sort(a,a+n,greater<int>());
		while(!a[n-1])--n;
		sort(a,a+n);
		int sum=accumulate(a,a+n,0ll);
		REP(i,0,n+1)REP(j,0,sum+1)f[i][j]=0;
		f[0][0]=1;
		int s=0;
		REP(i,0,n){
			REP(l,0,s+1)if(f[i][l]){
				REP(j,1,a[i]+1)if(j<=s+1){
					REP(k,0,l+1)if(k<=j){
						int w=l-k+a[i]-j;
                        f[i+1][w]=min(inf,f[i+1][w]+mul(mul(f[i][l],binom(l,k)),mul(binom(s-l+1,j-k),binom(a[i]-1,j-1))));
					}else break;
				}else break;
			}
			s+=a[i];
		}
//		REP(i,0,n+1){
//			REP(j,0,n*n)cout<<f[i][j]<<' ';
//			cout<<endl;
//		}
		return f[n][0];
	}
}s;
int k,n;
vector<vector<int>>b;
vector<int>a;
int res[400005][10];
void dfs(int x){
	if(x==k){
		b.pb(a);
		int p=b.size()-1;
		vector<int>t(k,0);
		REP(i,0,k)if(a[i])t[i]=s.solve(a,i);
		if(accumulate(all(a),0)==0){
			REP(i,0,k+1)res[p][i]=1;
			return;
		}
		REP(i,0,k+1){
			REP(j,0,k)if(i!=j)res[p][i]=min(t[j]+res[p][i],inf);
		}
		return;
	}
	REP(i,0,x+2)a[x]=i,dfs(x+1);
}
int c;
int asker(vector<int>t,int r){
//	cout<<'!';
//	for(auto i:t)cout<<i<<' ';
//	cout<<endl;
	if(r==k)return s.solve(t);
	++t[r];
	return s.solve(t,r);
	int sum=0;
//	int x=lbound(b,t);
//	cout<<r<<':'<<res[x][r]<<endl;
//	return res[x][r];
}
int ask(vector<int>t){
	//给定一个序列当作前缀，求方案数
	vector<int>a(k,0);
	vector<int>v=t;deal(v);
//	for(auto i:t)cout<<i<<' ';
//	cout<<endl;
	if(v.size()>k)return 0;
	int m=k-v.size();
	int co=binom(26-c-v.size(),m),sum=0;
	REP(i,0,m)co=mul(co,i+1);
	vector<int>b(k,26),pos(26,0);
	REP(i,0,v.size())b[i]=v[i];
	do{
		REP(i,0,k)if(b[i]!=26)pos[b[i]]=i;
		iota(all(a),1);
		for(auto i:t)--a[pos[i]];
		bool fl=1;
		REP(i,0,k)if(a[i]<0)fl=0;
		if(!fl)continue;
		int lst=t.size()? pos[t[t.size()-1]]:k;
		sum=min(inf,sum+asker(a,lst));
	}while(next_permutation(all(b)));
//	for(auto i:t)cout<<i<<' ';
//	cout<<endl;
//	cout<<sum<<'*'<<co<<endl;
	return mul(sum,co);
}
void Main() {
    C[0][0]=1;
    REP(i,1,600){
        C[i][0]=C[i][i]=1;
        REP(j,1,i)C[i][j]=min(inf,C[i-1][j-1]+C[i-1][j]);
    }
	cin>>k>>n;
    vector<int>A(k,0);iota(all(A),1);int x=s.solve(A);REP(i,0,k)x=mul(x,26-i);if(x<n)over(-1)
	int lst=-1;
	while(k>8){
		lst=c;
		cout<<char(c+97);
		REP(i,1,k)cout<<(char)(c+98)<<(char)(c+97);
		k-=2;c+=2;
	}
//	cout<<endl;
	a=vector<int>(k,0);
//	dfs(0);
//	cout<<'!'<<k<<endl;
//	cerr<<clock()<<endl;
	vector<int>cur;
	REP(i,0,k*(k+1)/2){
		cur.pb(0);
		REP(j,0,26-c)if(lst!=j+c){
//			cout<<j<<' ';cout.flush();
			cur[i]=j;
			int t=ask(cur);
//			cout<<t<<endl;
			if(n>t)n-=t;
			else break;
		}
		lst=cur[i]+c;
//		cout<<(char)(lst+97)<<endl;
	}
	for(auto i:cur)cout<<char(i+c+97);
}
void TC() {
    int tc=1;
	while(tc--){
		Main();
		cout.flush();
	}
}
signed main() {
	return cin.tie(0),cout.tie(0),ios::sync_with_stdio(0),TC(),0;
}