def hikaku(A, B):
    lA = list(str(A))
    lB = list(str(B))
    if len(lA) > len(lB):
        return A
    elif len(lA) < len(lB):
        return B
    else:
        N = len(lA)
        for i in range(N):
            if (lA[i] == '4') and (lB[i] == '7'):
                lA[i] = '7'
                lB[i] = '4'
            elif (lA[i] == '7') and (lB[i] == '4'):
                lA[i] = '4'
                lB[i] = '7'
        sA = ''
        sB = ''
        for j in range(N):
            sA = sA + lA[j]
            sB = sB + lB[j]
        cnvA = int(sA)
        cnvB = int(sB)
        if cnvA > cnvB:
            return A
        else:
            return B

def main():
    A, B = map(int, input().split())
    print(hikaku(A, B))

if __name__ == '__main__':
    main()