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

#include <bits/stdc++.h>
using namespace std;
int n, a[200010];
vector<int> v;
inline int gcd(int x, int y) {
  if (!y) return x;
  return gcd(y, x % y);
}
inline int read(int &x) {
  char ch;
  int N = 1;
  while ((ch = getchar()) && (!((ch >= '0') && (ch <= '9'))) && (!(ch == '-')))
    ;
  if ((ch == '-')) {
    N = -1;
    while ((ch = getchar()) && (!((ch >= '0') && (ch <= '9'))))
      ;
  }
  x = ch - 48;
  while ((ch = getchar()) && (((ch >= '0') && (ch <= '9'))))
    x = x * 10 + ch - 48;
  x *= N;
}
inline int Next(int x) {
  v.clear();
  for (int i = x; (i <= n) && (v.size() < 2); i++)
    if (a[i] != -1) v.push_back(i);
  if (v.size() < 2) return n + 1;
  int T1 = v[0], T2 = v[1];
  int A1 = a[T1], A2 = a[T2];
  if ((A2 - A1) % (T2 - T1)) return T2;
  int d = (A2 - A1) / (T2 - T1);
  int s = A2 - (T2 - x) * d;
  if (s <= 0) return T2;
  int Nw = 0;
  for (Nw = T2 + 1; Nw <= n; Nw++) {
    A2 += d;
    if ((A2 <= 0) || ((a[Nw] != -1) && (a[Nw] != A2))) break;
  }
  return Nw;
}
int main() {
  read(n);
  for (int i = 1; i <= n; i++) read(a[i]);
  int Cur = 1, Ans = 0;
  while (Cur <= n) Ans++, Cur = Next(Cur);
  printf("%d\n", Ans);
}
