# coding: utf-8
# Here your code !
input()
nums = list(map(int, input().split()))
nums_uniq = list(set(nums))
flag = False
nums.sort()
nums_uniq.sort()
if nums == nums_uniq:
    diff=abs(nums[0]-nums[1])
    curNum=nums[1]
    flag = True
    for i in nums:
        if abs(curNum-i)==diff:
            curNum = i
        else:
            flag=False
            break
if flag:
    print("YES")
else:
    print("NO")