MATLAB: How to randomly increase a random element in a matrix by one 'popping'?

I am stuck on a question and don't know how to go about it:

Write a function that given a matrix M of numbers representing locations and the number of popcorn pieces that have previously popped at each, returns a matrix with one more popped piece randomly added. In addition, the function returns the row-column coordinate where the latest piece popped.

For example:

>> a = zeros(2)
a = 
  0  0
  0  0
>> aa = popcorn(a)
aa = 
  1  0
  0  0
>>[aaa rc] = popcorn(aa)
aaa
  2  0
  0  0
rc =
  1  1
>> [aaaa rc] = popcorn(aaa)
aaaa = 
  2  0
  0  1
rc = 
  2  2
>>aaaaa = popcorn(aaaa)
aaaaa = 
  2  1
  0  1

So far, what i have is:

function output  = popcorn(M)
[w, z] = size(M);
a = randi(w);
f = randi(z);
b = w(a,f);
output = b;

But then i get an error saying:

Attempted to access w(1,2); index out of bounds because numel(w)=1.

Error in popcorn (line 5)
b = w(a,f);

And its here where I'm stuck, any help will be greatly appreciated!