Formula For A Calculation With Varying Spreadsheet

I've tried an if statement with AND function with no luck either.

my spreadsheet looks like this:
A1 = "One", B1 = 17/08/2013, C1 = 1000.00
A2 = "Two", B2 = 16/08/2013, C2 = 500.00
A3 = "One", B3 = 17/07/2013, C3 = 1500.00
A4 = "Two", B4 = 13/07/2013, C4 = 400.00
A5 = "Two", B5 = 14/07/2013, C5 = 300.00

"One" represents the total that needs to be paid and "Two" represents the payments.

What I want to do is recognise when "One" is present and deduct "two" from that amount, but the whole spreadsheet is never the same (the data is being pasted from somewhere else) so it's really, really difficult to do that.

So I want to do =1000-500 (for that period, so in D2 it should say 500) and then =1500-400-300 (for the other period so in D5 it should say 800).

NB: Any payment ("two") that comes in after the 17th of the month will be carried forward to the next months "one".