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;
long long num[200020];
const long long inf = 1LL << 60;
struct pro {
  int sz;
  long long d, ci, pi;
  vector<long long> vect;
  void init() {
    sz = 0;
    ci = inf;
    pi = inf;
    vect.clear();
    d = 0;
  }
  int size() { return sz; }
  bool add(pair<int, long long> c) {
    if (ci == inf && c.second != -1) {
      ci = c.first;
      sz++;
      vect.push_back(c.second);
      return true;
    } else if (pi == inf && c.second != -1) {
      pi = ci;
      ci = c.first;
      if ((num[ci] - num[pi]) % (ci - pi) != 0)
        return false;
      else {
        d = (num[ci] - num[pi]) / (ci - pi);
        for (int i = vect.size() - 1; i >= 0; i--) {
          long long curr = num[ci] - (vect.size() - 1 - i) * d;
          if (curr <= 0) return false;
        }
        sz++;
        vect.push_back(c.second);
        return true;
      }
    } else if (c.second != -1) {
      if ((num[c.first] - num[ci]) % (c.first - ci) != 0 ||
          (num[c.first] - num[ci]) / (c.first - ci) != d) {
        return false;
      } else {
        pi = ci;
        ci = c.first;
        sz++;
        vect.push_back(c.second);
        return true;
      }
    } else {
      if (pi == inf) {
        sz++;
        vect.push_back(c.second);
        return true;
      }
      long long tmp = num[c.first - 1] + d;
      if (tmp <= 0) return false;
      num[c.first] = tmp;
      sz++;
      vect.push_back(c.second);
      return true;
    }
  }
};
int main() {
  int n;
  cin >> n;
  pro p;
  p.init();
  int res = 0;
  for (int i = 0; i < n; i++) cin >> num[i];
  for (int i = 0; i < n; i++) {
    bool rec = p.add(make_pair(i, num[i]));
    if (!rec) {
      p.init();
      p.add(make_pair(i, num[i]));
      res++;
    }
  }
  if (p.size()) res++;
  cout << res << endl;
}
