'''****** Задание ********
  *Хайпанём немножечко!
'''
n = int(input())
num_false_h = -1
pre_h = 0
for i in range(n):
    block = int(input())
    h = block % 256
    m = block // 256 ** 2
    r = (block // 256) % 256
    true_h = (37 * (m + r + pre_h)) % 256
    pre_h = h
    if true_h >= 100 or true_h != h:
        num_false_h = i
        break
print(num_false_h)