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;
bool valid(int l, int r, vector<int> &arr, vector<int> &non_zero_index) {
  if (non_zero_index.size() <= 1) return true;
  if (non_zero_index.size() == 3) {
    int a = non_zero_index[0];
    int b = non_zero_index[1];
    int c = non_zero_index[2];
    int add = (arr[b] - arr[a]) / (b - a);
    if (arr[b] + (long long)(c - b) * add != (long long)arr[c])
      return false;
    else
      non_zero_index.pop_back();
  }
  int a = non_zero_index[0];
  int b = non_zero_index[1];
  int add = (arr[b] - arr[a]) / (b - a);
  if (arr[a] + (long long)(b - a) * add != (long long)arr[b]) return false;
  if (arr[a] - (long long)(a - l) * add <= 0) return false;
  if (arr[b] + (long long)(r - b) * add <= 0) return false;
  return true;
}
int main() {
  scanf("%d", &n);
  if (n <= 2) {
    cout << "1\n";
    return 0;
  }
  vector<int> arr(n);
  for (int i = 0; i < n; ++i) {
    int a;
    scanf("%d", &a);
    arr[i] = a;
    if (a == -1) arr[i] = 0;
  }
  int ans = 1, l = 0;
  vector<int> non_zero_index;
  for (int r = 0; r < n; ++r) {
    if (arr[r] != 0) non_zero_index.push_back(r);
    if (valid(l, r, arr, non_zero_index))
      continue;
    else {
      ans++;
      l = r;
      non_zero_index.resize(0);
      if (arr[r] != 0) non_zero_index.push_back(r);
    }
  }
  cout << ans << '\n';
}
