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>
const int maxn = 999;
using namespace std;
int n;
int tot = 0;
vector<int> w;
vector<int> s;
int p[99999];
void readd() {
  for (int i = 0; i < n; i++) {
    int d;
    cin >> d;
    p[i] = d;
    if (d != -1) {
      s.push_back(d);
      w.push_back(i);
    }
  }
}
void countt() {
  int od = maxn;
  if (s.size() > 0) int r = 0;
  int f;
  int d0;
  int rear = 0;
  int ok = 0;
  if (s.size() != 0)
    while (rear < s.size() - 1) {
      if (ok == 0) {
        int wd = w[rear + 1] - w[rear];
        int sd = s[rear + 1] - s[rear];
        if (sd % wd == 0) {
          ok = 1;
          int dd = sd / wd;
          d0 = dd;
          f = w[rear];
          for (int i = w[rear] + 1; i < w[rear + 1]; i++) {
            int now = p[f] + (i - f) * d0;
            if (p[i] == -1) p[i] = now;
          }
        } else {
          ok = 0;
          tot++;
        }
      } else {
        int wd = w[rear + 1] - f;
        int sd = s[rear + 1] - p[f];
        if (sd % wd == 0 && sd / wd == d0) {
          for (int i = w[rear] + 1; i < w[rear + 1]; i++) {
            int now = p[f] + (i - f) * d0;
            if (p[i] == -1) p[i] = now;
          }
        } else {
          ok = 0;
          tot++;
          int l = f - 1;
          int r = w[rear] + 1;
          while (l >= 0 && p[l] == -1) {
            p[l] = (l - f) * d0 + p[f];
            l--;
          }
          while (r < w[rear] && p[r] == -1) {
            p[r] = (r - f) * d0 + p[f];
            r++;
          }
        }
      }
      rear++;
    }
}
int main() {
  cin >> n;
  readd();
  countt();
  int i = 0;
  while (i < n) {
    while (p[i] != -1 && i < n) i++;
    while (p[i] == -1 && i < n) i++;
    tot++;
  }
  printf("%d", tot);
  return 0;
}
