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 maxn = 2e5 + 233;
const int inf = 2e9 + 10;
int f[maxn], a[maxn], pre[maxn];
int main() {
  int n;
  cin >> n;
  for (int i = 1; i <= n; i++) scanf("%d", &a[i]);
  for (int i = 1; i <= n; i++) pre[i] = maxn;
  a[0] = a[n + 1] = -1;
  int k = n;
  while (k >= 1) {
    long long buf = inf;
    int start = k, first = -1;
    while ((buf == inf || ((a[k] == -1 && a[first] + buf * (k - first) > 0) ||
                           a[k] == a[first] + buf * (k - first))) &&
           k) {
      if (a[k] != -1 && buf == inf) {
        if (first == -1)
          first = k;
        else {
          if ((a[first] - a[k]) % (first - k))
            break;
          else {
            buf = (a[first] - a[k]) / (first - k);
            int laji = a[first] + buf * (start - first);
            if (laji <= 0) break;
          }
        }
      }
      k--;
    }
    for (int i = k + 1; i <= start; i++) pre[i] = min(pre[i], k + 1);
  }
  k = 1;
  while (k <= n) {
    long long buf = inf;
    int start = k, first = -1;
    while ((buf == inf || ((a[k] == -1 && a[first] - buf * (first - k) > 0) ||
                           a[k] == a[first] - buf * (first - k))) &&
           k <= n) {
      if (a[k] != -1 && buf == inf) {
        if (first == -1)
          first = k;
        else {
          if ((a[k] - a[first]) % (k - first))
            break;
          else {
            buf = (a[k] - a[first]) / (k - first);
            int laji = a[first] - buf * (first - start);
            if (laji <= 0) break;
          }
        }
      }
      k++;
    }
    for (int i = start; i < k; i++) pre[i] = min(pre[i], start);
  }
  f[0] = 0;
  for (int i = 1; i <= n; i++) f[i] = f[pre[i] - 1] + 1;
  cout << f[n] << endl;
  return 0;
}
