Image processing with Numpy

hello everyone

i am having an issue with numpy doing the following m = np.array([[20, 30, 120, 110.110],
[30, 30, 130, 120,110],
[20, 50, 130, 140,130],
[40, 30, 140, 150,120],
[50, 40, 150, 140,130]])

prewitt = np.array([[1, 1, 1], [0, 0, 0], [-1, -1, -1]])

res = np.dot(prewitt,m)

i want to 20 1 + 30 1 + 20 1 + 30 0 +30 0+ 50 0 + 120*-1 + 130* -1 + 130* -1 = the total is going to be in a new array of 4 by 4 so on and so furth

i tried numpy matrix array but did not work