Leaderboard unknown global 'player' ?

How can I identify "player" on XP.Changed function?

local Level = Instance.new("IntValue")
Level.Name = "Level"
Level.Value = 1
Level.Parent = stats

local XP = Instance.new("IntValue")
XP.Name = "XP"
XP.Value = 0
XP.Parent = stats

local XPN = Instance.new("IntValue")
XPN.Name = "XPN"
XPN.Value = 500
XPN.Parent = stats

local Gold = Instance.new("IntValue")
Gold.Name = "Gold"
Gold.Value = 0
Gold.Parent = stats

XP.Changed:connect(function()
    if player:WaitForChild("XP").Value >= player:WaitForChild("XPN").Value then
        Level.Value = Level.Value+1
        XPN.Value = XPN.Value+500
    end
end)

end)