Python这个题应该怎么编写呢

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n = int(input())
sum = 0.0
for i in range(1,n+1):
    if i%2==0:
        sum-=n*(n-1)/((n+1)*(n+3))
    else:
        sum+=n*(n-1)/((n+1)*(n+3))
print("%.5f" % sum)