How to make a symmetric matrix from asymmetric matrix by averaging two values

Hello experts,

I have an asymmetric matrix like below (this is an example and the actual one has 50 columns and 50 lows).

A B C D
1 100 20 10 5
2 24 100 60 2
3 20 50 100 4
4 1 2 6 100

From the matrix above, I would like to get an average of two values in diagonal direction (e.g. A2&B1, A3&C1) and make a symmetric matrix like below. Is there any way to do it for many rows and columns? Any advice or help would be greatly appreciated.

A B C D
1 100 22 15 3
2 22 100 55 3
3 15 55 100 5
4 3 3 5 100