#coding: utf-8
#yuki_406

n=int(raw_input())
x=map(int,raw_input().split())

x.sort()
d=x[1]-x[0]
res=1
for i in xrange(n-1):
 if (x[i+1]-x[i])!=d or x[i+1]==x[i]:
  res=0
  break

if res==1:
 print "YES"
else:
 print "NO"