Pick numbers from a matrix and create a row vector

Hi if i have a matrix with input: [1 -2 3 4;3 2 -1 0;5 6 2 1] I want to take the corner elements of this matrix and create a row vector using them called v. I've tried it but keep getting errors, my code is below.

A=input('Give a matrix: '), A;
[m, n]=size(A);

v = [A(1,1), A(1,m), A(n,1), A(m, n)];
[x, y] = size(v);
disp(v)

It seems to work if i only have A(1,1) but not when i start using specific values. The initial matrix is meant to be user inputted.

[Mitul Dattani]

Nevermind figured it out i need to change where i used m and n to end and it works.