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;
const int Inf = 1e9 + 1;
const int N = 2e5 + 1;
int a[N], ne[N];
vector<int> v;
int main() {
  ios::sync_with_stdio(false);
  cin.tie(NULL);
  int n;
  cin >> n;
  for (int i = 0; i < n; i++) cin >> a[i];
  ne[n - 1] = n - 1;
  for (int i = n - 2; i > -1; i--) {
    if (a[i + 1] != -1)
      ne[i] = i + 1;
    else
      ne[i] = ne[i + 1];
  }
  int ans = 0;
  for (int i = 0; i < n;) {
    int f = i, s = i, j = i, d = Inf;
    v.clear();
    while (j < n) {
      if (a[j] != -1) v.push_back(j);
      int len = v.size();
      if (len >= 2 &&
          (a[v[len - 1]] - a[v[len - 2]]) % (v[len - 1] - v[len - 2])) {
        i = v[len - 1];
        break;
      }
      if (len >= 2) {
        int d = (a[v[len - 1]] - a[v[len - 2]]) / (v[len - 1] - v[len - 2]);
        if (len >= 3) {
          int dd = (a[v[len - 2]] - a[v[len - 3]]) / (v[len - 2] - v[len - 3]);
          if (d != dd) {
            i = v[len - 1];
            break;
          }
        }
        if (d > 0 && v[0] - (a[v[0]] - 1) / d > i) {
          i = v[len - 1];
          break;
        }
        if (d < 0 &&
            (a[v[len - 1]] - 1) / abs(d) + v[len - 1] < ne[v[len - 1]]) {
          i = a[v[len - 1]] - 1 / abs(d) + v[len - 1] + 1;
          break;
        }
      }
      j++;
    }
    ans++;
    if (j == n) break;
  }
  cout << ans << endl;
}
