def big(A,B):
    nA,nB=len(A),len(B)
    if nA>nB:   return A
    if nB>nA:   return B
    for i in range(nA):
        if A[i]=='4' and B[i]=='7': return A
        if A[i]=='7' and B[i]=='4': return B
        if A[i]>B[i]:               return A
        if A[i]<B[i]:               return B

A,B=input().split()
print(big(A,B))