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

"""
Codeforces Round 241 Div 1 Problem D

Author  : chaotic_iak
Language: Python 3.3.4
"""

class InputHandlerObject(object):
    inputs = []

    def getInput(self, n = 0):
        res = ""
        inputs = self.inputs
        if not inputs: inputs.extend(input().split(" "))
        if n == 0:
            res = inputs[:]
            inputs[:] = []
        while n > len(inputs):
            inputs.extend(input().split(" "))
        if n > 0:
            res = inputs[:n]
            inputs[:n] = []
        return res
InputHandler = InputHandlerObject()
g = InputHandler.getInput

############################## SOLUTION ##############################
n = int(input().strip())
q = [int(x) for x in g()]

diffs = []
indices = []
ct = 0
stillstart = True
for i in range(len(q)):
    if q[i] == 0: q[i] = -1 # because it's easier to type 0 than -1
    if q[i] == -1 and stillstart: continue
    if stillstart:
        stillstart = False
        ct = 1
        indices.append(i)
        continue
    if q[i] == -1:
        ct += 1
    else:
        d = q[i] - q[i-ct]
        if d % ct:
            diffs.append("X")
        else:
            diffs.append(d//ct)
        indices.append(i)
        ct = 1

if not diffs:
    print(1)
else:

    res = 1
    if diffs[0] != "X" and q[indices[0]] - diffs[0] * indices[0] <= 0:
        res += 1
        diffs.pop(0)
        indices.pop(0)

    prevok = False
    for i in range(len(diffs)):
        if diffs[i] == "X":
            res += 1
            notenough = False
            if 0 < i < len(diffs)-1:
                if diffs[i-1] != "X" and diffs[i+1] != "X":
                    leftmx = n if diffs[i-1] >= 0 else (- q[indices[i]] // diffs[i-1] + indices[i])
                    if diffs[i-1] < 0 and not q[indices[i]] % diffs[i-1]: leftmx -= 1
                    rightmn = 0 if diffs[i+1] <= 0 else (- q[indices[i+1]] // diffs[i+1] + indices[i+1])
                    if diffs[i+1] > 0 and not q[indices[i+1]] % diffs[i+1]: rightmn += 1
                    if leftmx + 1 < rightmn: notenough = True
            if notenough:
                res += 1
            prevok = False
            continue
        if prevok and diffs[i-1] == diffs[i]:
            continue
        if prevok:
            res += 1
            notenough = False
            if 0 < i < len(diffs)-1:
                if diffs[i-1] != "X" and diffs[i+1] != "X":
                    leftmx = n if diffs[i-1] >= 0 else (- q[indices[i]] // diffs[i-1] + indices[i])
                    if diffs[i-1] < 0 and not q[indices[i]] % diffs[i-1]: leftmx -= 1
                    rightmn = 0 if diffs[i+1] <= 0 else (- q[indices[i+1]] // diffs[i+1] + indices[i+1])
                    if diffs[i+1] > 0 and not q[indices[i+1]] % diffs[i+1]: rightmn += 1
                    if leftmx + 1 < rightmn: notenough = True
            if notenough:
                passed = True
                if i > 1:
                    i -= 1
                    if diffs[i-1] != "X" and diffs[i+1] != "X":
                        leftmx = n if diffs[i-1] >= 0 else (- q[indices[i]] // diffs[i-1] + indices[i])
                        if diffs[i-1] < 0 and not q[indices[i]] % diffs[i-1]: leftmx -= 1
                        rightmn = 0 if diffs[i+1] <= 0 else (- q[indices[i+1]] // diffs[i+1] + indices[i+1])
                        if diffs[i+1] > 0 and not q[indices[i+1]] % diffs[i+1]: rightmn += 1
                        if leftmx + 1 < rightmn: notenough = True
                    i += 1
                if i < len(diffs) - 2:
                    i += 1
                    if diffs[i-1] != "X" and diffs[i+1] != "X":
                        leftmx = n if diffs[i-1] >= 0 else (- q[indices[i]] // diffs[i-1] + indices[i])
                        if diffs[i-1] < 0 and not q[indices[i]] % diffs[i-1]: leftmx -= 1
                        rightmn = 0 if diffs[i+1] <= 0 else (- q[indices[i+1]] // diffs[i+1] + indices[i+1])
                        if diffs[i+1] > 0 and not q[indices[i+1]] % diffs[i+1]: rightmn += 1
                        if leftmx + 1 < rightmn: notenough = True
                    i -= 1
                if not passed:
                    res += 1
                    prevok = False
                else:
                    prevok = True
            else:
                prevok = False
            continue
        if not prevok:
            prevok = True
    if prevok and diffs[-1] != "X" and q[indices[-1]] + diffs[-1] * (n-1 - indices[-1]) <= 0: res += 1

    print(res)