#include <iostream>
#include <string>
#include <vector>
#include <cmath>
#include <set>
#include <map>
#include <algorithm>
#include <memory.h>
#include <stack>
#include <math.h>
#include <limits>
#include <sstream>
#include <cmath>
#ifndef _DEBUG
#define __int64 long long
#endif
using namespace std;

struct vertex {
	vertex* next[2];
	char msk;
	char c;
	vertex* p;
	vertex() {
		msk = 0;
		next[0] = 0;
		next[1] = 0;
		p = 0;
	}
};
vertex *root;

vertex keka[16000000];
int sz = 0;
int mak = 1;
void build(int l, int r, char *c) {
	vertex* cur = root;
	while (l < r) {
		cur->msk |= mak;
		if (cur->next[c[l] - '0']) {
			cur = cur->next[c[l] - '0'];
			++l;
			cur->msk |= mak;
			continue;
		}
		cur->next[c[l] - '0'] = keka + sz++;
		cur->next[c[l] - '0']->p = cur;
		cur = cur->next[c[l] - '0'];
		cur->msk |= mak;

		cur->c = c[l];
		++l;
	}

}
int k;
int kda = 0;
vertex* curas = 0;
void dfs(vertex* v) {

	if (v->msk != 3)
		return;
	if (kda == k) {
		kda++;
		curas = v;
		return;
	}
	kda++;

	if (v->next[0])
		dfs(v->next[0]);
	if (v->next[1])
		dfs(v->next[1]);
	return;
}


int main() {
#ifdef _DEBUG
	freopen("test.in", "r", stdin);
#else
	freopen("input.txt", "r", stdin);
	freopen("output.txt", "w", stdout);
#endif
	//ios::sync_with_stdio(0);
	char st1[5000], st2[5000];
	cin >> st1 >> st2;

	cin >> k;
	root = new vertex;
	int len = strlen(st1);
	for (int i = 0; i < len; ++i)
		build(i, len, st1);
	mak = 2;
	len = strlen(st2);
	for (int i = 0; i < len; ++i)
		build(i, len, st2);

	dfs(root);
	string st;
	while (curas != root) {
		st.push_back(curas->c);
		curas = curas->p;
	}
	reverse(st.begin(), st.end());
	cout << st << endl;
	return 0;
}