Subtract From Set Figure In One Cell Once Another Cell Total Reaches Zero

formula which uses 4 cells

the 4 cells are

Cell B2 - this is a set figure which is the initial base figure - currently set to 43
Cell C2 - This subtracts Cell B2 figure (which is 43) from Cell E34 [=SUM(B2-E34)]
Cell E34 - Is a cumulative total of 4 cells [=SUM(E4,E12,E19,E26)] and displays the total.
Cell D2 is a set figure of 49

The formula I am looking for is once C2 reaches zero, I need C2 to remain displaying zero and the remaining figure to be deducted from D2 and displayed in that cell so C2 figure would descend from 43 until it reaches zero. but would then start decreasing D2 whilst C2 would remain static at zero and carry that the remaining figure to D2 which would be deducted from the starting total of 49 and display the figure.