Solution for 'lazy' in PlaidCTF 2015

lazy.sage

from ast import literal_eval

ciphertext = int(open("ciphertext.txt", 'rb').read())
pubkey = literal_eval(open("pubkey.txt", 'rb').read())

def mat(pubkey, ciphertext, B):
    n = len(pubkey)
    A = Matrix(ZZ,n+1,n+1)
    for i in range(n):
        A[i,i] = 1
    for i in range(n):
        A[i,n] = B*pubkey[i]
    A[n,n] = -B*ciphertext
    return A

def check(v):
    return all(0 <= x <= 1 for x in v)

n = len(pubkey)
B = int(ceil(sqrt(n * 2**n)))
A = mat(pubkey, ciphertext, B)
print "Computing LLL..."
res = A.LLL()
sol = res[-3]
offset = 1
bits = [int(bool(x)) for x in sol][::-1][offset:]
res = ""
for i in range(0, len(bits), 8):
    b = "".join(str(x) for x in bits[i:i+8])
    b = "01" + b[2:]
    res += chr(int(b,2))
print "Flag:", res

What does B stand for here?
Also, I tried for B = 1, didn't work.