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;
vector<long long> a;
vector<pair<long long, int> > done;
int dp[300000][2][2];
int sol(int i, int j, int k) {
  if (i >= ((int)done.size()) - 1) return 0;
  if (dp[i][j][k] != -1) return dp[i][j][k];
  int mn = 10000000;
  if (j == 1) {
    long long d = done[i].first - done[i - 1].first;
    d /= (done[i].second - done[i - 1].second);
    long long d2 = done[i + 1].first - done[i].first;
    long long dis = (done[i + 1].second - done[i].second);
    if (d2 % dis == 0) {
      d2 /= dis;
      if (d2 == d) mn = min(mn, sol(i + 1, 1, 1));
    }
    if ((dis - 1) * d + done[i].first > 0)
      mn = min(mn, 1 + sol(i + 1, 0, 1));
    else
      mn = min(mn, 1 + sol(i + 1, 0, 0));
  } else {
    long long d2 = done[i + 1].first - done[i].first;
    long long dis = (done[i + 1].second - done[i].second);
    if (d2 % dis == 0) {
      d2 /= dis;
      long long dis2;
      if (i) {
        dis2 = done[i].second - done[i - 1].second - 1;
      } else
        dis2 = done[i].second;
      if (done[i].first - (d2 * dis2) > 0 || k == 1)
        mn = min(mn, sol(i + 1, 1, 1));
      else
        mn = min(mn, 1 + sol(i + 1, 1, 1));
    }
    mn = min(mn, 1 + sol(i + 1, 0, 1));
  }
  return dp[i][j][k] = mn;
}
int main() {
  int n;
  cin >> n;
  a = vector<long long>(n);
  for (int i = 0; i < n; i++) cin >> a[i];
  for (int i = 0; i < n; i++) {
    if (a[i] != -1) {
      done.push_back(make_pair(a[i], i));
    }
  }
  for (int i = 0; i < 300000; i++) {
    dp[i][0][0] = dp[i][0][1] = dp[i][1][0] = dp[i][1][1] = -1;
  }
  cout << 1 + sol(0, 0, 0) << endl;
}
