def main():
    import sys
    S = sys.stdin.readline().strip()
    T = sys.stdin.readline().strip()

    # Check if there's any character in S not present in T
    t_chars = set(T)
    for c in 'abcdefghijklmnopqrstuvwxyz':
        if c in S and c not in t_chars:
            print(c)
            return

    # Preprocess T for subsequence checks
    n_t = len(T)
    next_pos = [[n_t] * 26 for _ in range(n_t + 1)]
    for i in range(n_t - 1, -1, -1):
        for c in range(26):
            next_pos[i][c] = next_pos[i + 1][c]
        current_ord = ord(T[i]) - ord('a')
        next_pos[i][current_ord] = i

    # Check if S is a subsequence of T
    current_t = 0
    for c in S:
        c_ord = ord(c) - ord('a')
        pos = next_pos[current_t][c_ord]
        if pos >= n_t:
            print(S)
            return
        current_t = pos + 1

    # If S is a subsequence of T
    print(-1)

if __name__ == "__main__":
    main()