n = int(input())
a = [int(input()) for x in range(n)]
d = a[1] - a[0]
print(d if str(a) == str([*range(a[0], a[0] + d*n, d)]) else 0)
biA9IGludChpbnB1dCgpKQphID0gW2ludChpbnB1dCgpKSBmb3IgeCBpbiByYW5nZShuKV0KZCA9IGFbMV0gLSBhWzBdCnByaW50KGQgaWYgc3RyKGEpID09IHN0cihbKnJhbmdlKGFbMF0sIGFbMF0gKyBkKm4sIGQpXSkgZWxzZSAwKQ==