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 math
from fractions import gcd

class State:
    res = 0
    start = 0        
    dif = None
    firstpos = None

    def restart(self, cur, seq):
        self.res = self.res + 1
        self.start = cur
        self.dif = None
        self.firstpos = None

    def value_at(self, pos, seq):
        if self.firstpos != None and self.dif != None:
            return seq[self.firstpos] + (pos - self.firstpos) * self.dif 
        return None

    def print_state(self):
        print self.res,self.start,self.dif,self.firstpos

def main():
    n = int(raw_input().strip())
    seq = [int(j) for j in raw_input().strip().split(" ")]

    s = State()
    s.restart(0, seq)

    for cur in range(len(seq)):
        if s.firstpos == None and seq[cur] >= 0:
            s.firstpos = cur

        if seq[cur] == -1:
            v = s.value_at(cur, seq)
            if v != None and v <= 0:
                s.restart(cur, seq)
            continue

        if cur == s.start:
            continue
        
        dist = cur - s.firstpos

        if s.dif == None and dist > 0:
            if (seq[cur] - seq[s.firstpos]) % dist == 0:
                s.dif = (seq[cur] - seq[s.firstpos]) / dist
                v = s.value_at(s.start, seq)
                if v != None and v <= 0:
                    s.restart(cur, seq)
                    s.firstpos = cur
            else:
                s.restart(cur, seq)
            continue

        v = s.value_at(cur, seq)
        if v != None and v != seq[cur]:
            s.restart(cur, seq)

    print s.res
    return 0

if __name__ == "__main__":
    main()
