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;
int n, ps, pe;
long long ds, de, a[200010];
int st;
bool isde() {
  int pt;
  bool F = false;
  if (ps > 0 && st == 1 && (a[pe] - a[ps]) % (pe - ps) == 0) {
    de = (a[pe] - a[ps]) / (pe - ps);
    F = true;
  } else {
    for (int i = pe + 1; i <= n; i++) {
      if (a[i] != -1) {
        pt = i;
        F = true;
        break;
      }
    }
    if (F && (a[pt] - a[pe]) % (pt - pe) == 0 &&
        (a[pe] - (pe - ps - 1) * (a[pt] - a[pe]) / (pt - pe)) > 0) {
      de = (a[pt] - a[pe]) / (pt - pe);
    } else
      de = 0, F = false;
  }
  return F;
}
int ans() {
  int cnt = 0;
  long long d = 0;
  for (int i = 1; i < n; i++) {
    cnt++;
    d = a[i] - a[i + 1];
    for (; i < n && a[i] - a[i + 1] == d; i++)
      ;
  }
  return cnt + (a[n - 1] - a[n] != d);
}
int main() {
  cin >> n;
  for (int i = 1; i <= n; i++) cin >> a[i];
  a[n + 1] = -1;
  ps = 0, pe = 0, ds = 0, de = 0;
  st = 0;
  while (ps < n) {
    while (a[ps + 1] != -1) {
      ps++;
      if (st > 1) {
        if (a[ps] - a[ps - 1] != a[ps - 1] - a[ps - 2]) st = 0, ds = 0;
      } else
        ds = a[ps] - a[ps - 1];
      st++;
    }
    if (ps < n) {
      pe = ps + 1;
      while (pe <= n && a[pe] == -1) pe++;
      switch (st) {
        case 0:
          if (pe == n + 1) {
            for (int i = pe - 1; i > ps; i--) a[i] = 1;
          } else {
            isde();
            for (int i = pe - 1; i > ps; i--) a[i] = a[i + 1] - de;
            de = 0;
          }
          break;
        case 1:
          if (isde()) {
            for (int i = pe - 1; i > ps; i--) a[i] = a[i + 1] - de;
            break;
          }
        default:
          for (int i = ps + 1; i < pe && a[i - 1] + ds > 0; i++) {
            a[i] = a[i - 1] + ds;
            ps = i;
          }
          if (ps < pe - 1) {
            st = 0;
            ds = 0;
          }
      }
    }
  }
  cout << ans();
  return 0;
}
