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 main() {
  int n;
  scanf("%d", &n);
  int arr[n + 10];
  for (int i = 1; i <= n; i++) scanf("%d", &arr[i]);
  int start, end;
  start = 1;
  int pos_1 = -1, pos_2 = -1;
  int cnt = 0;
  int t = 0;
  for (int i = 1; i <= n;) {
    pos_1 = -1;
    pos_2 = -1;
    start = i;
    for (int j = start; j <= n; j++) {
      if (arr[j] != -1) {
        pos_1 = j;
        break;
      }
    }
    if (pos_1 != -1) {
      for (int j = pos_1 + 1; j <= n; j++)
        if (arr[j] != -1) {
          pos_2 = j;
          break;
        }
      if (pos_2 == -1) {
        cnt++;
        break;
      }
      t++;
      if (arr[pos_1] == arr[pos_2]) {
        start = pos_2 + 1;
        for (int j = pos_2 + 1; j <= n; j++)
          if (arr[j] == -1 || arr[j] == arr[pos_1])
            start = j + 1;
          else {
            start = j;
            break;
          }
        cnt++;
      } else {
        long long val = (arr[pos_2] - arr[pos_1]);
        int d = pos_2 - pos_1;
        if (val % d == 0) {
          int yes = val / d;
          val = arr[pos_1];
          yes *= (-1);
          for (int j = pos_1 - 1; j >= start; j--) {
            val += yes;
            if (val <= 0) {
              cnt++;
              start = pos_2;
              goto End;
            }
            if (arr[j] != -1 && arr[j] != val) {
              cnt++;
              start = pos_2;
              goto End;
            }
          }
          val = arr[pos_2];
          yes *= (-1);
          start = -1;
          for (int j = pos_2 + 1; j <= n; j++) {
            val += yes;
            if (val <= 0) {
              start = j;
              break;
            }
            if (arr[j] != -1 && arr[j] != val) {
              start = j;
              break;
            }
          }
          if (start == -1) start = n + 1;
          cnt++;
        } else {
          cnt += 1;
          start = pos_2;
        }
      }
    } else {
      cnt++;
      break;
    }
  End:;
    i = start;
  }
  cout << cnt << endl;
  return 0;
}
