import sys

sys.setrecursionlimit(200005)
int1 = lambda x: int(x)-1
p2D = lambda x: print(*x, sep="\n")
def II(): return int(sys.stdin.readline())
def LI(): return list(map(int, sys.stdin.readline().split()))
def LI1(): return list(map(int1, sys.stdin.readline().split()))
def LLI(rows_number): return [LI() for _ in range(rows_number)]
def LLI1(rows_number): return [LI1() for _ in range(rows_number)]
def SI(): return sys.stdin.readline().rstrip()
# dij = [(0, 1), (-1, 0), (0, -1), (1, 0)]
dij = [(0, 1), (-1, 0), (0, -1), (1, 0), (1, 1), (1, -1), (-1, 1), (-1, -1)]
inf = 10**16
md = 998244353
# md = 10**9+7

s = SI()
t = SI()
m, n = len(s), len(t)

# sub[i]=j...s[i:]はt[j:]の部分文字列
sub = [-1]*m
j = n-1
for i in range(m-1, -1, -1):
    while j >= 0 and s[i] != t[j]:
        j -= 1
    sub[i] = j
    j -= 1
# print(sub)

def maketo(s):
    res = []
    now = [-1]*26
    for i in range(len(s)-1, -1, -1):
        res.append(now[:])
        c = ord(s[i])-97
        now[c] = i
    res.append(now[:])
    return res[::-1]

tos = maketo(s)
tot = maketo(t)
# print(tos, tot)

def dfs(i=0, j=0):
    if i == m: return ""
    if j <= sub[i]: return ""
    for c in range(26):
        if tos[i][c] == -1: continue
        if tot[j][c] == -1: return chr(c+97)
        ret = dfs(tos[i][c]+1, tot[j][c]+1)
        if ret == "": continue
        return chr(c+97)+ret
    return ""

ans = dfs()
if ans == "": print(-1)
else: print(ans)