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 MAXN = 2e5 + 100;
const int INF = 1e9 + 100;
int n;
int mas[MAXN];
vector<pair<int, int>> vec;
int main() {
  ios::sync_with_stdio(false);
  cout.tie(0);
  cin.tie(0);
  cin >> n;
  for (int i = 0; i < n; ++i) {
    cin >> mas[i];
    if (mas[i] != -1) {
      vec.push_back({mas[i], i});
    }
  }
  if (vec.size() == 0) {
    cout << 1 << endl;
    return 0;
  }
  vector<pair<int, int>> fans;
  int k = vec.size();
  for (int i = 0; i < k - 1; ++i) {
    int j = i + 1;
    if ((vec[j].first - vec[i].first) % (vec[j].second - vec[i].second) != 0) {
      fans.push_back({i, j});
      continue;
    }
    int d = (vec[j].first - vec[i].first) / (vec[j].second - vec[i].second);
    for (; j < k; ++j) {
      if (((vec[j].first - vec[i].first) % (vec[j].second - vec[i].second) !=
           0) ||
          ((vec[j].first - vec[i].first) / (vec[j].second - vec[i].second) !=
           d)) {
        break;
      }
    }
    fans.push_back({i, j});
  }
  fans.push_back({k - 1, k});
  reverse(fans.begin(), fans.end());
  int dp[MAXN];
  fill(dp, dp + n, INF);
  dp[n] = 0;
  int l, r;
  for (auto e : fans) {
    if (e.first == 0) {
      l = 0;
    } else {
      l = vec[e.first - 1].second + 1;
    }
    if (e.second == k) {
      r = n;
    } else {
      r = vec[e.second].second;
    }
    for (int i = l; i < r; ++i) {
      dp[i] = min(dp[i], dp[r] + 1);
    }
  }
  cout << dp[0] << endl;
  return 0;
}
