Derangement.py

import itertools
import math
import sys

count = 0
n = int(sys.argv[1])

for left in itertools.permutations(range(1, n + 1)):
    for right in itertools.permutations(range(1, n + 1)):
        rIsGood = all(i + 1 != r for i, r in enumerate(right))
        lIsGood = all(i + 1 != l for i, l in enumerate(left))
        if rIsGood or lIsGood:
            print left, right
            count += 1

print count, math.factorial(n)**2
print count / float(math.factorial(n)**2)