#include <bits/stdc++.h>
using namespace std;
#define rep(i,a,n) for (int i=a;i<n;i++)
#define per(i,a,n) for (int i=n-1;i>=a;i--)
#define pb push_back
#define eb emplace_back
#define mp make_pair
#define all(x) (x).begin(),(x).end()
#define fi first
#define se second
#define SZ(x) ((int)(x).size())
typedef vector<int> VI;
typedef basic_string<int> BI;
typedef long long ll;
typedef pair<int,int> PII;
typedef double db;
mt19937 mrand(random_device{}()); 
const ll mod=1000000007;
int rnd(int x) { return mrand() % x;}
ll powmod(ll a,ll b) {ll res=1;a%=mod; assert(b>=0); for(;b;b>>=1){if(b&1)res=res*a%mod;a=a*a%mod;}return res;}
ll gcd(ll a,ll b) { return b?gcd(b,a%b):a;}
// head

const int N=201000;
const int M=26;
struct node {
	node *go[27],*p;
	node *ngo[27];
	vector<node*> son;
	int val,plen;
	ll sub,rsub;
};

struct sam {
	node* newnode() {
		node *p=cur++;
		p->val=p->plen=0; p->p=0;
		memset(p->go,0,sizeof(p->go));
		p->son.clear();
		return p;
	}
	node *append(node *p,int w) {
		node *np=newnode();np->val=p->val+1;
		for (;p&&!p->go[w];p=p->p) p->go[w]=np;
		if (!p) np->p=rt;
		else {
			node *q=p->go[w];
			if (q->val==p->val+1) np->p=q;
			else {
				node *nq=newnode();
				nq->val=p->val+1;
				memcpy(nq->go,q->go,sizeof(q->go));
				nq->p=q->p;
				np->p=q->p=nq;
				for (;p&&p->go[w]==q;p=p->p) p->go[w]=nq;
			}
		}
		return np;
	}
	ll countdag(node *p) {
		if (p->sub!=0) return p->sub;
		p->sub=1;
		rep(i,0,M) {
			if (p->go[i]) p->ngo[i]=p;
			else {
				if (p->p) p->ngo[i]=p->p->ngo[i];
				else p->ngo[i]=NULL;
			}
		}
		rep(i,0,M) if (p->go[i]) p->sub+=countdag(p->go[i]);
		return p->sub;
	}
	void dfs(node *p,int sl) {
		int sr=p->val;
		p->plen=sl;
		rep(i,0,SZ(p->son)) {
			p->son[i]->rsub=p->rsub+(sr-sl)*p->sub;
			dfs(p->son[i],sr);
		}
	}
	void init() {
		n=strlen(s+1);
		cur=pool;
		rt=newnode();
		node *np=rt;
		rep(i,1,n+1) {
			np=append(np,s[i]-'a');
		}
		for (node *p=pool;p!=cur;p++) if (p->p) p->p->son.pb(p);
		countdag(rt);
		dfs(rt,-1);
	}
	ll eval(node *p,int len) {
		return p->rsub+(len-p->plen)*p->sub;
	}
	void nextgo(node* &p,int w,int &len) {
		if (p->go[w]) p=p->go[w],++len;
		else {
			if (p->ngo[w]==NULL) p=rt,len=0;
			else {
				p=p->ngo[w]; len=p->val;
				p=p->go[w]; ++len;
			}
		}
	}
	node pool[N],*cur,*rt;
	char s[N];
	int n;
}s,t;

ll k;
struct state {
	node *s,*t,*t2;
	int slen,lent,lent2,totlen;
};
ll eval(state ps) {
	ll way=0;
	if (ps.totlen==-1) return 0;
	if (ps.s!=NULL) {
		way+=(ps.s->sub-(ps.s==s.rt))*(t.rt->sub-1);
		way+=t.eval(ps.t,ps.lent)-t.rt->sub;
		if (ps.lent==ps.totlen&&ps.totlen!=0) way-=ps.t->sub;
		return way;
	} else {
		if (ps.slen==0) return 0;
		if (ps.lent<ps.totlen-ps.slen-1) return 0;
		way=t.eval(ps.t,ps.lent)-t.eval(ps.t2,ps.totlen-ps.slen-1);
		if (ps.lent==ps.totlen) way-=ps.t->sub;
		return way;
	}
}
state trans(state ps,int w) {
	if (ps.totlen==-1) return ps;
	++ps.totlen;
	if (ps.s==NULL) {
		t.nextgo(ps.t,w,ps.lent);
		if (ps.lent+ps.slen<ps.totlen) {
			ps.totlen=-1;
			return ps;
		}
		assert(ps.t2->go[w]!=NULL);
		ps.t2=ps.t2->go[w];
	} else {
		if (ps.s->go[w]) {
			ps.s=ps.s->go[w];
			t.nextgo(ps.t,w,ps.lent);
		} else {
			ps.s=NULL;
			ps.slen=ps.totlen-1;
			t.nextgo(ps.t,w,ps.lent);
			if (ps.lent+ps.slen<ps.totlen) {
				ps.totlen=-1;
				return ps;
			}
			ps.t2=t.rt;
		}
	}
	return ps;
}
state ps;
ll cway[30];
int main() {
	scanf("%s%s",s.s+1,t.s+1);
	s.init(); t.init();
	scanf("%lld",&k);
	if (k>(s.rt->sub-1)*(t.rt->sub-1)) {
		puts("-1");
	}
	ps.s=s.rt; ps.t=t.rt;
	string ans;
	/*auto x=trans(ps,1);
	x=trans(x,1);
	printf("!! %lld\n",eval(x));*/
	while (1) {
		ll way=eval(ps);
		//puts("gao");
		//puts(ans.c_str());
		rep(i,0,M) {
			auto qs=trans(ps,i);
			cway[i]=eval(qs);
			//printf("--- %d %d\n",i,cway[i]);
			way-=cway[i];
		}
		if (way>=k) {
			break;
		}
		k-=way;
		rep(i,0,M) {
			if (cway[i]>=k) {
				ps=trans(ps,i);
				ans.pb(i+'a');
				break;
			} else {
				k-=cway[i];
			}
		}
	}
	puts(ans.c_str());
}