print((function (f, n)
	local a,t = f()
	if t==0 then return "NO" end
	for i=2,n-1 do if t~=a[i+1]-a[i] then return "NO" end end
	return "YES"
end)(function ()
	local a = {}
	for e in io.stdin:read("*l"):gmatch("%d+") do table.insert(a,tonumber(e)) end
	table.sort(a)
	return a,a[2]-a[1]
end, tonumber(io.stdin:read("*l"))))