3840.sagews

x = 0
n = 1000000
k = 20000
for y in range(10001):
    x = x + binomial(n,y) * binomial(n-y,k-2*y) / (2^y * n^k)

r = x * factorial(k)

__SAGE__ KeyboardInterrupt __SAGE__

print "result is %e"%r

result is 2.688280e-01