//#define _GLIBCXX_DEBUG
#define _CRT_SECURE_NO_WARNINGS
#include <bits/stdc++.h>

using namespace std;

typedef long long ll;
typedef pair<int, int> pii;
typedef vector<int> vi;
typedef long double ld;

#define mk make_pair
#define inb push_back
#define X first
#define Y second
#define all(v) v.begin(), v.end()
#define sqr(x) (x) * (x)
#define TIME 1.0 * clock() / CLOCKS_PER_SEC
#define y1 AYDARBOG
//continue break pop_back return

int solve();

int main()
{
	//ios_base::sync_with_stdio(0);
	//cin.tie(0);
#define TASK "password"
#ifndef _DEBUG
	freopen(TASK".in", "r", stdin), freopen(TASK".out", "w", stdout);
#endif
	solve();
#ifdef _DEBUG
	fprintf(stderr, "\nTIME: %.3f\n", TIME);
#endif
}

/*const int BUFSZ = (int)1e5 + 7;
char buf[BUFSZ];
string get_str()
{
	scanf(" %s", buf);
	return string(buf);
}*/

int cnt[26], cntsk[26];
char s[30000001], t[3000001];
char ans[3000001];
int sz = 0;

int solve()
{
	scanf(" %s", s);
	scanf(" %s", t);
	int n = strlen(s);
	int m = strlen(t);
	for (int i = 0; i < n; ++i)
	{
		cntsk[s[i] - 'a']++;
	}
	for (int i = 0; i < m; ++i)
	{
		cnt[t[i] - 'a']++, cntsk[t[i] - 'a']--;
	}
	for (int i = 0; i < 26; ++i)
		if (cntsk[i] < 0)
			puts("impossible"), exit(0);
	for (int i = 0; i < n; ++i)
	{
		int cur = s[i] - 'a';
		if (cnt[cur])
		{
			while (sz && cntsk[ans[sz - 1] - 'a'] && s[i] < ans[sz - 1])
			{
				--cntsk[ans[sz - 1] - 'a'];
				++cnt[ans[sz - 1] - 'a'];
				--sz;
			}
			ans[sz] = s[i];
			++sz;
			--cnt[ans[sz - 1] - 'a'];
		}
		else
		{
			--cntsk[cur];
		}
	}
	ans[sz] = 0;
	printf("%s\n", ans);
	return 0;
}