Submission #188002

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Source Code Expand

n,d=map(int,raw_input().split())
x,y=map(int,raw_input().split())

def c(n,r):
	if n < 0 or r < 0 or r > n:
		return 0
	ret = 1.0
	for i in xrange(r):
		ret = ret * (n-i) / (i+1)
	return ret

x = abs(x)
y = abs(y)
if x % d != 0 or y % d != 0:
	print 0.0
else:
	x /= d
	y /= d
	if n < x+y or (x+y+n) % 2 != 0:
		print 0.0
	else:
		import math
		a1 = math.log(c(n,(n-x+y)/2))
		a2 = math.log(c(n,(n+x+y)/2))
		a3 = n*math.log(4.0)
		print "{0:.12f}".format(math.exp(a1+a2-a3))

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