[Python] Diagonal difference in Python

I am working on a HackerRank problem and I don't understand some of the logic:

if (i == j):
left += a[j]


The above is saying if the row/column indices are the same([1,1], [2,2], [3,3]), append value found at those coordinates to the list 'left'

I don't understand the logic in the code below. To me it looks like it's saying append values found at the coordinates where row index + column index = 3 (n-1) but I don't think that's right. What would the code below translate to?

if (i + j) == (n - 1):
right += a[j]


Below is the function with sample inputs a and n.

a = [[ 1, 2, 3, 4 ],
[ 5, 6, 7, 8 ],
[ 1, 2, 3, 4 ],
[ 5, 6, 7, 9 ]]

n = 4
def xsum(a, n):
left = 0
right = 0
for i in range(0, n):
for j in range(0, n):
if (i == j):
left += a[j]
if (i + j) == (n - 1):
right += a[j]
return (abs(left-right))

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