You have a random array which contains a height of walls.
The aim is to count how many empty space can be filled.
Imagine, that empty space will be filled by grains and it means that if one of the wall's side will be lower then another, it will be overflowed.
It's not necessary, that wall can be on the edges, count of them can be any and they can be side by side.
Example of array: [0, 7, 2, 9, 0, 4, 6, 7]