Calculate the difference between first and last entry in a row?

Hi,

Please open the attachment and help me out.

In row D (on the last line of the entry ABC 123) i need to calculate the difference between the first and last entry i.e. D3 =C3-C1, until the value in row A changes. If there is only one row, I just need a zero.
How do I do this easily, with a macro or a function? I have a lagre number of rows, and doing it manually (with D3 =C3-C1 etc.) will take forever...

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Hopefully somebody understands... Thanks in advance.

// Teo