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, in[200005];
bool is_pro(int a, int b, int c, int xa, int xb, int xc) {
  if (((a - b) % (xa - xb)) || ((a - c) % (xa - xc)) || ((c - b) % (xc - xb)))
    return 0;
  if ((a - b) / (xa - xb) != (b - c) / (xb - xc)) return 0;
  return 1;
}
int solve() {
  vector<int> b;
  for (int i = 0; i < n; i++)
    if (in[i] != -1) b.push_back(i);
  if (b.size() == 0 || b.size() == 1) return 1;
  if (b.size() == 2) {
    if ((in[b[0]] - in[b[1]]) % (b[0] - b[1])) return 2;
    int cst = (in[b[0]] - in[b[1]]) / (b[0] - b[1]);
    for (int i = b[0] - 1; i >= 0; i--) {
      in[i] = in[i + 1] - cst;
      if (in[i] < 0) return 2;
    }
    for (int i = b[0] + 1; i < n; i++) {
      in[i] = in[i - 1] + cst;
      if (in[i] < 0) return 2;
    }
    return 1;
  }
  int ans = 0;
  for (int i = 1; i < b.size() - 1; i++) {
    ans += 1 - is_pro(in[b[i - 1]], in[b[i]], in[b[i + 1]], b[i - 1], b[i],
                      b[i + 1]);
  }
  cout << ans << endl;
  return ans;
}
int main() {
  cin >> n;
  for (int i = 0; i < n; i++) cin >> in[i];
  solve();
  return 0;
}
