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;
template <class T>
bool umin(T& a, T b) {
  if (a > b) {
    a = b;
    return 1;
  }
  return 0;
}
template <class T>
bool umax(T& a, T b) {
  if (a < b) {
    a = b;
    return 1;
  }
  return 0;
}
int n, ans, arr[200009];
int solve(int x) {
  int last = 1000000007, prog = 1000000007;
  for (int i = x; i <= n; i++) {
    if (arr[i] > 0) {
      if (last != 1000000007 and (arr[i] - arr[last]) % (i - last) != 0) {
        if (prog >= 0) return i;
        int cur = arr[last];
        for (int j = last + 1; j < i; j++) {
          cur += prog;
          if (cur < 1) return j;
        }
        return i;
      }
      if (prog != 1000000007 and prog != (arr[i] - arr[last]) / (i - last)) {
        if (prog >= 0) return i;
        int cur = arr[last];
        for (int j = last + 1; j < i; j++) {
          cur += prog;
          if (cur < 1) return j;
        }
        return i;
      }
      if (last != 1000000007) {
        if (prog == 1000000007) {
          prog = (arr[i] - arr[last]) / (i - last);
          int st = arr[last] - prog * (last - x);
          if (st < 1) return last + 1;
        } else
          prog = (arr[i] - arr[last]) / (i - last);
      }
      last = i;
      continue;
    }
  }
  if (prog != 1000000007) {
    int cur = arr[last];
    for (int j = last + 1; j <= n; j++) {
      cur += prog;
      if (cur < 1) return j;
    }
  }
  return n + 1;
}
int main() {
  scanf("%d", &n);
  for (int i = 1; i <= n; i++) scanf("%d", arr + i);
  if (*max_element(arr + 1, arr + n + 1) == -1) return 0 * puts("0");
  int p = 1;
  while (p <= n) {
    p = solve(p);
    ans++;
  }
  printf("%d\n", ans);
  return 0;
}
