N = int(input())
xlist = input().split()
xlist = [int(i) for i in xlist]
xlist.sort()

distlist = [0 for i in range(N)]

for i in range(0, N-1):
    distlist[i] = xlist[i+1] - xlist[i]
temp = distlist[0]

flag = 1
for i in range(0, N-1):
    if distlist[i] == 0:
        flag = 0
    elif distlist[i] == temp:
        pass
    else:
        flag = 0

if flag == 1:
    print("YES")
elif flag == 0:
    print("No")
else:
    print("ERROR")