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>
long long n, a[200005], lst, lc, ln = 1, ans = 1, p1, p2, t, llp[200005], fr;
bool setd;
int max(int x, int y) { return x > y ? x : y; }
void find(int st) {
  p1 = 1;
  p2 = 1;
  bool c1 = 0;
  for (int i = st; i < n; i++) {
    if (a[i] != -1 && !c1) {
      p1 = i + 1;
      c1 = 1;
      continue;
    }
    if (a[i] != -1 && c1) {
      p2 = i + 1;
      break;
    }
  }
}
bool check(int st, long long o) {
  int p = st - 1;
  if (a[st] != -1 && st > 0) p--;
  while (p >= fr && a[p] == -1) p--;
  p++;
  if (p == st - 1 || a[p] != -1) return 1;
  if (a[st] - o * (st - p) <= 0) return 0;
  return 1;
}
int lp(int st) {
  if (llp[st]) return llp[st];
  long long ct = 0;
  for (int i = st; i < n; i++) {
    if (a[i] != -1) {
      if (!ct)
        ct = a[i];
      else if (ct != a[i]) {
        llp[st] = i;
        return i;
      }
    }
  }
  llp[st] = n - 1;
  return n - 1;
}
int main() {
  scanf("%I64d", &n);
  for (int i = 0; i < n; i++) scanf("%I64d", a + i);
  find(0);
  lc = a[p2 - 1] - a[p1 - 1], ln = p2 - p1;
  if (ln == 0) {
    printf("%I64d", ans);
    return 0;
  }
  int cnt = 0;
  for (int i = 0; i < n; i++)
    if (a[i] != -1) cnt++;
  if (cnt == 1) {
    printf("%I64d", ans);
    return 0;
  }
  for (int i = 0; i < n; i++) {
    fr = max(fr, i);
    if (i < 0) continue;
    if (a[i] != -1) {
      if (i - lst == 0) continue;
      if (a[lst] != -1)
        t = a[i] - a[lst];
      else
        t = a[i];
      if (!setd || (t / (i - lst) == lc / ln && t % (i - lst) == 0)) {
        if (a[lst] != -1) lc = t, ln = i - lst;
        setd = 1;
        if ((a[p2 - 1] - a[p1 - 1]) % (p2 - p1) != 0 ||
            !check(i, (a[p2 - 1] - a[p1 - 1]) / (p2 - p1))) {
          int t = lp(i), oi = i;
          if (t > i + 1)
            i = t, ans++;
          else
            ans++, i++;
          find(i);
          lc = a[p2 - 1] - a[p1 - 1], ln = p2 - p1, lst = p1 - 1;
        }
      } else {
        find(i);
        lc = a[p2 - 1] - a[p1 - 1], ln = p2 - p1;
        ans++;
        if (ln == 0) break;
      }
      lst = i;
    } else {
      if (i != 0 && ((a[lst] != -1 && a[lst] + (i - lst) * (lc / ln) <= 0) ||
                     ((a[lst] == -1 && (i - lst) * (lc / ln) <= 0)))) {
        ans++;
        find(i);
        lc = a[p2 - 1] - a[p1 - 1], ln = p2 - p1, lst = p1 - 1;
        if (ln == 0) break;
        i = max(i, p1 - 1);
      }
    }
  }
  printf("%I64d", ans);
  return 0;
}
