Amount from lastest date minus the value from the oldest date

Hello,

i have some values in the body of my report which show by date values.

I would like to take the latest dates value and minus it from the oldest dates value.

e.g

date Amount

1/3/12 = 45

1/4/12 = 30

1/6/12 =65

1/21/12 = 89

basilcay 89 - 45 is what i am after.

I have tried so far

maximum(Amount) - minimum(Amount) the issue is that it is working off 89 - 30...

any ideas?