Merge cell arrays with special algorithm

Hello,

I need to create a different output cell aray, when I have input cell array. I have at input:

 in = { [1,1] =  100001
        [2,1] =  100000
        [3,1] =  100001
        [4,1] =  100000
        [1,2] = 8, 9, 10, 11, 12
        [2,2] = 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24
        [3,2] = 28, 29, 30, 31, 32
        [4,2] = 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72
}.

I need output cell at size (n,2), where n is the number of different values at first column - it means at this case, n = 2, beacuse we have '100001' and '100000'. When it would be e.g. '100000', '100001','100002', then n = 3, etc. At the second column at out, it should be a vectors from input(i,2) respectively fit to first column - I mean, all vectors from one number, should be concatenated to one big vector. So the output cell (in the discussed case) should be:

 out ={ [1,1] = 100000
        [2,1] = 100001
        [1,2] = [8, 9, 10, 11, 12 ,28, 29, 30, 31, 32]
        [2,2] = [13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72]
       }.

So the first column is a "title" of the second column, which contains a all numeric value from properly columns from input.

I know, I have described it not so clear, but maybe, it can be understand :D Thanks for help in advance, and if You have question to me, ask! Thanks!