while creating a array using numpy.arrange, how to negate only the last 50 numbers?

[vicky53]

I'm creating an array using numpy and trying to negate the numbers from 50 to 100 in seperate steps as below..
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arr = np.arange(100)
arr1 = arr[50:100]*-1
arr2=(arr[0:50])
result=np.concatenate([arr2,arr1])

Is it possible to negate the intended numbers in the first step itself?