N = int(raw_input())
duck = map(int, raw_input().split())
duck.sort()
tmp = duck[1]-duck[0]
ans = 'NO'
if tmp != 0:
	for i in range(1, N-1):
		if duck[i+1]-duck[i] != tmp: break
		if i == N-2: ans = 'YES'
print ans