from collections import Counter

def solution(X, Y):
    cntX = Counter(X)
    cntY = Counter(Y)
    
    ans = ""
    
    for k in "9876543210" :
        ans += (k * min(cntX[k], cntY[k]))
    
    if len(ans) == 0 :
        ans = "-1"
    elif ans[0] == "0" :
        ans = "0"
    
    return ans

# X = "5525"
# Y = "1255"
# print(solution(X, Y))