Formula to minus preivious figure if culomn A value is the same

[joeydeacan]

Hit there,

I have this problem which I have tried to sort but cant, Ive got 900 rows to do so manually is gonna take some time and my sanity.

I have a route number and a fuel cost, the only way i could get SQL to export was to give me the cost of each distance from base, but when they are on the same route it should be, 100, 25(not 125), 50(not 150)

So when the route number is the same minus the one above with the same route number, but also I dont know how to do anything that will work with the first entry i.e. The first entry should should just be as it is ?

I have posted an example of how it is and how it should be.

Can anyone help at all ?

[protonLeah]

=if(a2=a1,j2-j1,j2)