3
-1 1 -1
 4
45 -1 41 -1
 1
-1
 5
40 -1 44 46 48
 6
43 40 37 34 -1 -1
 7
-1 2 4 -1 4 1 5
 19
23 26 -1 -1 35 38 41 -1 -1 -1 53 -1 59 62 6 7 8 9 -1
 6
-1 2 6 -1 -1 6
 16
3 8 13 18 23 -1 -1 -1 43 48 53 45 -1 -1 -1 -1
 13
25 24 23 22 24 27 -1 33 -1 2 2 2 -1
 12
-1 17 -1 54 -1 64 -1 74 79 84 -1 94
 8
-1 12 14 16 18 20 -1 -1
 13
2 -1 3 1 3 1 -1 1 3 -1 -1 1 1
 5
-1 40 42 -1 46
 2
1000000000 -1
 14
-1 5 3 -1 -1 31 31 31 -1 31 -1 -1 4 7
 3
-1 1000000000 999999999
 7
32 33 34 -1 -1 37 38
 1
1000000000
 10
29 27 -1 23 42 -1 -1 45 -1 -1
 3
39 42 -1
 15
-1 28 -1 32 34 26 -1 26 -1 -1 26 26 26 -1 -1
 3
1000000000 999999999 1000000000
 17
-1 -1 -1 -1 64 68 72 -1 45 46 47 48 49 50 51 52 53
 3
-1 1 1000000000
 2
-1 1000000000
 3
999999999 -1 1000000000
 1
1
 2
1000000000 1000000000
 3
999999999 1000000000 -1
 7
11 8 5 -1 -1 -1 -1
 3
-1 1 2
 2
-1 21
 8
-1 -1 1 7 -1 9 5 2
 11
9 21 17 13 -1 -1 -1 -1 -1 -1 -1
 3
-1 1000000000 -1
 6
-1 6 1 -1 -1 -1
 1
65
 20
-1 32 37 -1 -1 -1 57 -1 -1 40 31 33 -1 -1 39 47 43 -1 35 32
 5
-1 1 7 -1 5
 18
21 19 -1 -1 -1 48 50 -1 54 -1 5 1 -1 -1 -1 37 36 35
 9
42 39 36 33 -1 -1 -1 34 39
 2
-1 -1

1
 1
 1
 1
 1
 3
 2
 2
 2
 3
 2
 1
 6
 1
 1
 3
 1
 1
 1
 2
 1
 2
 2
 2
 2
 1
 2
 1
 1
 1
 2
 2
 1
 3
 3
 1
 2
 1
 5
 2
 4
 2
 1

import sys

n = int(sys.stdin.readline())
if n <= 2:
  print(1)
  sys.exit(0)

a = [int(s) for s in sys.stdin.readline().split()]

# number of arithmetic progressions
seg = [-1 for i in range(0,n)]

# start and end positive number for a arithmetic progressions
# eg
# -1 6 -1 2 -1 [start=1 (the number 6), end=3 (the number 2)]
# 
st = [-1 for i in range(0, n)]
ed = [-1 for i in range(0, n)]

if a[0] > 0 and a[1] > 0:
  st[1] = 0
  ed[1] = 1
elif a[0] > 0:
  st[1] = 0
elif a[1] > 0:
  st[1] = 1

seg[0] = seg[1] = 1
#print(str(a[1]) + " : " + str(seg[1]) + " : " + str(st[1]) + " : " + str(ed[1]))

for i in range(2, n):
  if a[i] == -1:
    seg[i], st[i], ed[i] = seg[i-1], st[i-1], ed[i-1]
  else:
    if st[i-1] == -1: #find a start point
      seg[i], st[i], ed[i] = seg[i-1], i, ed[i-1]
    elif ed[i-1] == -1: #check a[st[i-1]] ... a[i] is a AP
      if (a[i] - a[st[i-1]]) % (i - st[i-1]) == 0:
        seg[i], st[i], ed[i] = seg[i-1], st[i-1], i
      else:
        seg[i], st[i], ed[i] = seg[i-1]+1, i, -1
    else:
      sum1 = a[ed[i-1]] - a[st[i-1]]
      sum2 = a[i] - a[ed[i-1]]
      if sum1 % (ed[i-1] - st[i-1]) == 0 and sum2 % (i - ed[i-1]) == 0 and \
         sum1 * (i - ed[i-1]) == sum2 * (ed[i-1] - st[i-1]):
        seg[i], st[i], ed[i] = seg[i-1], st[i-1], i
      else:
        seg[i], st[i], ed[i] = seg[i-1]+1, i, -1
  #print(str(a[i]) + " : " + str(seg[i]) + " : " + str(st[i]) + " : " + str(ed[i]))

print(seg[n-1])

