import sys

_ = input()
lst = list(map(int, input().split()))

lst.sort()

tmp = lst[1] - lst[0]

if tmp == 0:
    print('NO')
    sys.exit()

for t in range(len(lst) - 2):
    if lst[t + 2] - lst[t + 1] != tmp:
        print('NO')
        break
else:
    print('YES')