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 N = 200000 + 10;
int n;
long long a[N];
int getNext(int st) {
  long long tmp = st;
  vector<pair<long long, int> > vec;
  for (; tmp < n && vec.size() < 2; tmp++) {
    if (a[tmp] > 0) {
      vec.push_back(make_pair(a[tmp], tmp));
    }
  }
  if (vec.size() != 2) return n;
  long long v1 = vec[0].first, t1 = vec[0].second;
  long long v2 = vec[1].first, t2 = vec[1].second;
  if ((v2 - v1) % (t2 - t1) == 0) {
    long long d = (v2 - v1) / (t2 - t1);
    tmp = v1;
    int flag = true;
    for (int i = t1 - 1; i >= st; --i) {
      tmp -= d;
      if (tmp <= 0) {
        flag = false;
        break;
      }
    }
    if (flag) {
      st = t2;
      tmp = v2;
      for (; st < n && tmp > 0;) {
        if (a[st] > 0 && a[st] != tmp) break;
        ++st;
        tmp += d;
      }
      return st;
    }
  }
  return t2;
}
void solve() {
  cin >> n;
  for (int i = 0; i < n; ++i) {
    scanf("%I64d", a + i);
  }
  int ret = 0;
  int tmp = 0;
  for (; tmp != n;) {
    ++ret;
    tmp = getNext(tmp);
  }
  cout << ret << endl;
}
int main() {
  solve();
  return 0;
}
