def is_subsequence(s, t):
    it = iter(t)
    return all(c in it for c in s)

S = input().strip()
T = input().strip()

if not S:
    print(-1)
    exit()

# Find the lexicographically smallest character in S
min_char = min(S)
# Check if this character exists in T
if min_char not in T:
    print(min_char)
    exit()

# Check if the entire S is a subsequence of T
if is_subsequence(S, T):
    print(-1)
else:
    print(S)