n = int(input())
ls = list(map(int, input().split()))
sls = sorted(ls)

flag = 0
for i in range(n-2):
    if sls[i+1] - sls[i] == sls[i+2] - sls[i+1]:
        if sls[i+1] - sls[i] ==0:
            flag = 1
        else:
            None
    else:
        flag = 1
        break
if flag == 0:
    print('YES')
else:
    print('NO')