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

#sys.stdin = open('in.txt')
R = lambda: list(map(int, input().split()))

n = int(input())

a = R()
pre = a[:]
INF = 10000000000

def solve():
    ans = 1
    global a, n
    delta = INF
    for i in range(1, n):
        if a[i] != -1 and a[i - 1] != -1 and a[i] - a[i - 1] != delta:
            if delta != INF:
                ans += 1
                delta = INF
            else:
                delta = a[i] - a[i - 1]
        elif a[i] == -1 and delta != INF:
            if a[i - 1] + delta <= 0:
                ans += 1
                delta = INF
            else:
                a[i] = a[i - 1] + delta
        elif a[i] == -1 and delta == INF:
            if a[i - 1] == -1:
                pass
            else:
                j = i
                for j in range(i + 1, n):
                    if a[j] != -1:
                        if abs(a[i - 1] - a[j]) % (j - i + 1) == 0:
                            delta = (a[j] - a[i - 1]) // (j - i + 1)
                            for k in range(j-1, i-1, -1):
                                a[k]=a[k+1]-delta
                        else:
                            ans += 1
                            delta = INF
                        break

    if a[0]==-1:
        for i in range(n):
            if a[i]!=-1:
                if i<n-1 and a[i+1]!=-1:
                    d = a[i+1]-a[i]
                    fst = a[i]-d*i
                    if fst<=0:
                        ans+=1
                else:
                    pass

                break

    return ans

ans = solve()
a = pre[::-1]

ans=min(solve(), ans)


print(ans)
