def is_subsequence(s, t):
    i = j = 0
    len_s = len(s)
    len_t = len(t)
    while i < len_s and j < len_t:
        if s[i] == t[j]:
            i += 1
        j += 1
    return i == len_s

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

if not is_subsequence(S, T):
    print(S)
else:
    i = 0
    n = len(S)
    m = len(T)
    prefix_matched = [False] * (n + 1)
    prefix_matched[0] = True
    j = 0
    while i < n and j < m:
        if S[i] == T[j]:
            i += 1
            prefix_matched[i] = True
        j += 1
    
    for k in range(1, n + 1):
        if not prefix_matched[k]:
            print(S[:k])
            exit()
    
    print(-1)