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 = 200005;
int a[MAXN];
int he[MAXN];
int b[MAXN];
int n;
int call() {
  int ans = 0, sz = 0, cnt = 0;
  for (int i = 1; i <= n; i++) {
    if (a[i] != -1) {
      a[++sz] = a[i];
      b[sz] = cnt;
      cnt = 0;
    } else
      cnt++;
  }
  if (sz == 0) {
    printf("1\n");
    return 0;
  }
  for (int i = 1; i <= sz;) {
    if (i == sz) {
      if (min(b[i], cnt) < a[i])
        ans++;
      else
        ans += 2;
      break;
    }
    if (a[i] == a[i + 1]) {
      ans++;
      i++;
      while (i < sz && a[i] == a[i + 1]) i++;
      i++;
      b[i] = 0;
    } else if (a[i] < a[i + 1]) {
      int val = a[i + 1] - a[i];
      int mo = b[i + 1] + 1;
      if (val % mo != 0) {
        ans++;
        if (b[i] < a[i]) {
          i++;
          b[i] = 0;
          continue;
        }
        i++;
        if (i > sz) {
          cnt = cnt - min(a[i - 1] - 1, cnt);
          if (cnt > 0) ans++;
          break;
        }
        b[i] = b[i] - min(a[i - 1] - 1, b[i]);
      } else {
        ans++;
        int cha = val / mo;
        if (a[i] - (long long)cha * b[i] > 0) {
          i++;
          while (i < sz && (a[i + 1] - a[i]) % (b[i + 1] + 1) == 0 &&
                 (a[i + 1] - a[i]) / (b[i + 1] + 1) == cha)
            i++;
          i++;
          b[i] = 0;
          continue;
        }
        i++;
        if (i > sz) {
          cnt = cnt - min(a[i - 1] - 1, cnt);
          if (cnt > 0) ans++;
          break;
        }
        b[i] = b[i] - min(a[i - 1] - 1, b[i]);
      }
    } else {
      int val = a[i] - a[i + 1];
      int mo = b[i + 1] + 1;
      if (val % mo != 0) {
        ans++;
        if (b[i] < a[i]) {
          i++;
          b[i] = 0;
          continue;
        }
        i++;
        if (i > sz) {
          cnt = cnt - min(a[i - 1] - 1, cnt);
          if (cnt > 0) ans++;
          break;
        }
        b[i] = b[i] - min(a[i - 1] - 1, b[i]);
      } else {
        ans++;
        int cha = val / mo;
        i++;
        while (i < sz && (a[i] - a[i + 1]) % (b[i + 1] + 1) == 0 &&
               (a[i] - a[i + 1]) / (b[i + 1] + 1) == cha)
          i++;
        i++;
        if (i > sz) {
          if (a[i - 1] <= (long long)cnt * cha) ans++;
          break;
        }
        b[i] = b[i] - min((a[i - 1] - 1) / cha, b[i]);
      }
    }
  }
  return ans;
}
int main() {
  scanf("%d", &n);
  for (int i = 1; i <= n; i++) scanf("%d", &he[i]);
  for (int i = 1; i <= n; i++) a[i] = he[i];
  int ans = call();
  for (int i = 1; i <= n; i++) a[i] = he[n - i + 1];
  ans = min(ans, call());
  printf("%d\n", ans);
  return 0;
}
