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[1000000];
bool check(int x, int y) {
  int j = -1, k = -1;
  bool b = 0;
  for (int i = x; i <= y; i++)
    if (a[i] != -1)
      if (!b)
        b = 1, j = i;
      else {
        k = i;
        break;
      }
  if (j == -1) return true;
  if (k == -1) return true;
  long long r;
  if (a[j] > a[k]) {
    r = fabs(a[j] - a[k]) / (k - j);
    long long fr = (j - x) * r + a[j];
    for (int i = x; i <= y; i++) {
      if (fr <= 0) return 0;
      if (a[i] == -1)
        fr -= r;
      else if (a[i] != fr)
        return 0;
      else
        fr -= r;
    }
    return 1;
  } else if (a[j] < a[k]) {
    r = fabs(a[j] - a[k]) / (k - j);
    long long fr = a[j] - (j - x) * r;
    for (int i = x; i <= y; i++) {
      if (fr <= 0) return 0;
      if (a[i] == -1)
        fr += r;
      else if (a[i] != fr)
        return 0;
      else
        fr += r;
    }
    return 1;
  } else {
    for (int i = x; i <= y; i++)
      if (a[i] != -1 && a[i] != a[j]) return 0;
    return 1;
  }
}
int main() {
  cin >> n;
  for (int i = 0; i < n; i++) cin >> a[i];
  int i = 0, t = 0;
  while (i < n) {
    int fr = i, last = n, mid, j;
    while (fr <= last) {
      mid = (fr + last) / 2;
      if (check(i, mid)) {
        fr = mid + 1;
        j = mid;
      } else {
        last = mid - 1;
      }
    }
    t++;
    i = j + 1;
  }
  cout << t;
  return 0;
}
