Help needed to understand this testcase from 729C

Problem:- Codeforces 729C
Can someone explain how this testcase evaluates to -1?

TestCase 26
1 1 1000000000 1000000000
1 1000000000


The distance between the starting point and the first gas station is 1.
The distance between the first gas station and the destination is 10^9 -1.
Both can be covered using the car with capacity 10^9.
Am I missing something?

Read the question carefully before trying to solve it…

I did understand my mistake. Thanks for the help btw.