In het openoffice calc forum heeft iemand het voor me opgelost: hier zijn suggestie in het engels
This is a bit tricky, because you can't be sure if you want a number in a cell or a calculation.
If you don't mind having three cells for data entry and other cells with the calculations, then it's easy. Assuming start in A1, end in A2, travelled in A3, then put in cells C1, C2, and C3:
=IF(COUNT($A$1:$A$3)=2;IF(ISBLANK(A1);A2-A3;A1);"N/A")
=IF(COUNT($A$1:$A$3)=2;IF(ISBLANK(A2);A1+A3;A2);"N/A")
=IF(COUNT($A$1:$A$3)=2;IF(ISBLANK(A3);A2-A1;A3);"N/A")
These work on the idea that if there are 2 values, then do a calculation if the relevant cell is blank or just show the value if it's already specified, but if there are not 2 values, then show N/A.
