array change

[svm]

Hi,

can you help me to understand where I'm wrong in coding?

a = [] # define first array (future matrix)
arr = [] # define a second array(matrix too)
while True:
    text = input()

    if text == 'end': # if typed end, then create matrix from inputed numbers
        break
    a.append([int(j) for j in text.split()]) # fullfill the matrix

for i in range(len(a)): 
    for k in range(len(a[i])):
        arr[i][k]= a[i][k+1] + a[i][k-1] + a[i+1][k] + a[i-1][k] # create new matrix as a sum of all nearby numbers
        k += 1
    i += 1
print(arr)