Python/ Simple ROT encryption

from string import maketrans   # Required to call maketrans function.

def rot_n()
    print 'Using positive numbers only'
    a = int(raw_input('Enter amount to ROT text by >\n'))

    intab = "abcdefghijklmnopqrstuvwxyz"
    outtab = intab[a:]+intab[:a]
    trantab = maketrans(intab, outtab)

    str1 = raw_input('Type in text to be ROTted >\n')
    str2 = str1.lower()
    print '\n------RESULT------\n'
    print str2.translate(trantab);

rot_n()

""" Simple ROT encryption """