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
import math

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

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

st = -1 # index of first positive number in current subset of a
ed = -1 # index last positive number in current subset of a 
        # differation is (a[ed] - a[st])/(ed - st)
leading_zeros = 0 # -1 before a[st]
seg_count = 1

for (i, v) in enumerate(a):
  if v == -1:
    if st == -1:
      leading_zeros += 1
    else:
      if ed != -1:
        # check if v should be a non-positive number
        if a[ed] + (i-ed) * (a[ed] - a[st])/(ed-st) <= 0:
          st = -1
          ed = -1
          leading_zeros = 1
          seg_count += 1
        else:
          pass
      else:
        pass
  else:
    if st == -1:
      st = i # find first positive number
    else:
      if ed == -1:
        #print(i)
        if (v - a[st]) % (i-st) != 0 or a[st] - (v-a[st])/(i-st) * leading_zeros <= 0:
          # a[st..i] can't be an arithmetic progression
          st = -1
          ed = -1
          seg_count += 1
          leading_zeros = 0
        else:
          ed = i
      else:
        if (v-a[ed]) * (ed - st) != (a[ed] - a[st]) * (i-ed):
          st = i
          ed = -1
          seg_count += 1
          leading_zeros = 0
        else:
          ed = i #leave ed the first positive number after a[st] is also ok
  #print( "[" +str(st) + " " + str(ed) + "] " + str(seg_count) + " " + str(leading_zeros) )

print(seg_count)