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

dif = ls[2] - ls[1]

if dif == 0:
    print('NO')
else:
    for i in range(2, len(ls) - 1):
        if ls[i + 1] - ls[i] != dif:
            print('NO')
            break
    else:
        print('YES')