gets
a=gets.split.map{|e| e.to_i}.sort
if (t=a[1]-a[0])>0 then
	i=-1
	puts (a.select{
		i=i+1;
		(i+1<a.size)&&(a[i+1]-a[i]==t)	
	}.count==a.size-1)?"YES":"NO"
else
	puts "NO"
end