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;
bool comp(pair<int, int> a, pair<int, int> b) { return a.first < b.first; }
long long gcd(long long a, long long b) {
  if (b == 0) return a;
  return gcd(b, a % b);
}
int main() {
  ios_base::sync_with_stdio(0);
  int n, tmp;
  cin >> n;
  vector<int> val;
  vector<int> ind;
  vector<int> mat;
  vector<int> div;
  for (int i = int(0); i < int(n); ++i) {
    cin >> tmp;
    if (tmp != -1) {
      val.push_back(tmp);
      ind.push_back(i);
      mat.push_back(-1);
      div.push_back(-1);
    }
  }
  int m = val.size();
  if (m == 0) {
    cout << 1 << endl;
    return 0;
  }
  vector<int> solnotmat(m, 0);
  vector<int> solmat(m, 0);
  solnotmat[0] = solmat[0] = 1;
  for (int i = int(1); i < int(m); ++i) {
    solnotmat[i] = min(solnotmat[i - 1], solmat[i - 1]) + 1;
    int newmat, newdiv;
    newmat = val[i] - val[i - 1];
    newdiv = ind[i] - ind[i - 1];
    int g = (int)gcd((long long)newmat, (long long)newdiv);
    newmat /= g;
    newdiv /= g;
    if ((mat[i - 1] == -1) || (newmat == mat[i - 1] && newdiv == div[i - 1])) {
      solmat[i] = solmat[i - 1];
      mat[i] = newmat;
      div[i] = newdiv;
    } else {
      solmat[i] = solnotmat[i];
    }
  }
  int sol = min(solmat[m - 1], solnotmat[m - 1]);
  cout << sol << endl;
  return 0;
}
