How to subtract the value on a IntValue?

Everytime i do this:

p = script.Parent.Bricks

while true do
wait(0.2)
script.Parent.Text = "There are" .. p.Value .. "% Of Bricks Left"
end

With another script:

while true do
wait(1)
script.Parent.Bricks.Value - 1
end

When i try to subtract the value it doesnt work! Am i doing something wrong?

It's a common mistake, here:

while true do
wait(1)
script.Parent.Bricks.Value = script.Parent.Bricks.Value - 1
end