How can I generate random numbers between -1 and 1 using Linear Congruent Generator (LCG) method?

The following source code generates random numbers between 0 and 1:

def nextRandom(seed):
    m = 233280 # modulus
    a = 9301 # multiplier
    c = 49297 # increment
    x0 = seed # start-value
    return ((a * x0 + c) % m)/m

What modification can I apply to this source code to generate random numbers between -1 and 1?