Lua CC redstone timer

a = 0
os.setComputerLabel()
while true do
if redstone.getInput("back") then
print(a)
os.setComputerLabel("".. a)
a = a + 0.1
sleep(0.1)
else
sleep(0.1)
if a > 0 then
os.setComputerLabel("Ended at "..a)
end
a = 0
end
end