Excel 2007 : Increasing the value of cell and comparison

Can somebody help me?
Please show me how to do something with IF function.
In row 4 i have sum of column B and column C.
I want to compare the cell B4 and C4.
I want to increase the value of cell C1, with adding the number 1 until the C4=B4


-------A-----B-----C
1-------------3-----1
2-------------5-----3
3-------------9-----4

4------------17----8 ----this is sum


=IF(C4<B4;C1+1;C4) ??

..with this way, the result is 2, because IF function do adding just one time...
..help...tnx...