I have attempted to make a stamina system and for some reason it's random?

I have attempted to make a stamina system but the problem is that whenever I start the game and look at the output it goes negative and in a second it'd be -1000 and whenever you start running it'd go up there is no part of the script that would do this

so that means the amount of time of running will take longer to deplete stamina if you don't run edit: if your wondering why I have renderstepped, there's code that is not part of the issue

local timeyouahavebeenrunning = 0
RunService.RenderStepped:Connect(function()
    local stamina = 6


    if timeyouahavebeenrunning == stamina then
        camera.FieldOfView = 68
        humanoid.WalkSpeed = 14
        wait(6)
        timeyouahavebeenrunning = timeyouahavebeenrunning - timeyouahavebeenrunning
    elseif Userinputservice:IsKeyDown(Enum.KeyCode.LeftShift) or Userinputservice:IsKeyDown(Enum.KeyCode.RightShift) then

            camera.FieldOfView = 80
            humanoid.WalkSpeed = 23

            repeat 

                timeyouahavebeenrunning = timeyouahavebeenrunning + 1
                wait(1)

            until timeyouahavebeenrunning == 6
        else
            camera.FieldOfView = 70

            humanoid.WalkSpeed = 16

            repeat 
                timeyouahavebeenrunning = timeyouahavebeenrunning - 1
                wait(1)

            until Userinputservice:IsKeyDown(Enum.KeyCode.LeftShift)or Userinputservice:IsKeyDown(Enum.KeyCode.RightShift) or timeyouahavebeenrunning == 0

            end
        end)