How to add property value

hi i have a problem with script. I want to make my car gears switch with buttons.

shiftu and shiftd is a keyboard sensors
shift is property

problem: when i press shift up property shift is 5.
I think that problem is because siftu is always positive (it should be positive 1 time per press). So Help me who can

code:

if shiftu.isPositive() and own.shift == 1:
own.shift += 1
if shiftu.isPositive() and own.shift == 2:
own.shift += 2
if shiftu.isPositive() and own.shift == 3:
own.shift += 1
if shiftu.isPositive() and own.shift == 4:
own.shift += 1
if shiftd.isPositive() and own.shift == 1:
own.shift -= 2
if shiftu.isPositive() and own.shift == -1:
own.shift += 2

or i explain simply:
how to add property value in python using keyboard:
if im hit button and script add just 1(not 1+1+1+1… like in pulse mode):eek:

my english bad sorry