30 units possible to carry
1 unit -> 1 cell

start with 30 unit of gas                         ( 0 30) 
go forward 10 put 10 go back with remainig 10 units                  ( 0  0)

go forward with 30 again spent 10 but get 10  (10 30)

go forward 6 spending 6 put 8                 (16 16)
go back to the base spending remaining 16 units            ( 0  0)

go forward 16 with 30 but spend 16 but get 8  (16 22)

then go forward and reach 38 units

Note that parameters here are highly artificially devised
to fit the problem.
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