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

import java.io.*;
import java.util.*;
  
public class Main implements Runnable {
  final boolean isFileIO = false; 
  BufferedReader in;
  PrintWriter out;
  StringTokenizer st = new StringTokenizer("");
  String delim = " ";

  public static void main(String[] args) {
    new Thread(null, new Main(), "", 268435456).start(); 
  }

  public void run() {
    try {
      initIO();
      solve();
      out.close();
    } catch(Exception e) {
      e.printStackTrace(System.err);
      System.exit(-1);
    }
  }

  public void initIO() throws IOException {
    if(!isFileIO) {
      in = new BufferedReader(new InputStreamReader(System.in));
      out = new PrintWriter(System.out);
    } else {
      in = new BufferedReader(new FileReader("input.txt"));
      out = new PrintWriter("output.txt"); 
    }
  }
  
  String nextToken() throws IOException {
    if(!st.hasMoreTokens()) {
      st = new StringTokenizer(in.readLine());
    }
    return st.nextToken(delim);
  }
 
  String readLine() throws IOException {
    return in.readLine();
  }

  int nextInt() throws IOException {
   return Integer.parseInt(nextToken());
  }

  long nextLong() throws IOException {
    return Long.parseLong(nextToken());
  }

  double nextDouble() throws IOException {
    return Double.parseDouble(nextToken());
  }

  int get(long[] a) {
    int n = a.length;
    long[] d = new long[n];
    int cnt = 0;
    int ind = 0;
    for(int i = 0; i < n; i++) {
      if(a[i] != -1) {
        ind = i;
      } else
        cnt++;
    }
    if(cnt == n) {
      return 1;
    }
    if(cnt == n - 1) {
      return 1;
    }
    int start = 0;
    for(int i = 0; i < n; i++)
      if(a[i] != - 1) {
        start = i;
        break;
      }
    int end = 0;
    for(int i = n - 1; i >= 0; i--) {
      if(a[i] != -1){
        end = i;
        break;
      }
    }
    int second = 0;
    for(int i = start + 1; i < n; i++)
      if(a[i] != -1) {
        second = i;
        break;
      }
    if((a[second] - a[start]) % (second - start) ==0)
      d[start] = (a[second] - a[start]) / (second - start);
    else
      d[start] = 0;
    for(int i = start, j = 0; i < end; ) {
      j = i + 1;
      for(; j <= end; j++) {
        if(a[j] != -1)
          break;
      }
      if(d[i] == 0) {
        if((a[j] - a[i]) % (j - i) == 0) {
          long v = (a[j] - a[i]) / (j - i);
          for(int z = i + 1; z < j; z++)
            a[z] = a[z - 1] + v;
          d[j] = v;
        } else {
          for(int z = i + 1; z < j; z++)
            a[z] = a[z - 1] + 1;
          d[j] = 0;
        }
      } else {
        if((a[j] - a[i]) == (j - i) * d[i]) {
          for(int z = i + 1; z < j; z++)
            a[z] = a[z - 1] + d[i];
          d[j] = d[i];
        } else {
          for(int z = i + 1; z < j; z++) {
            a[z] = a[z - 1] + d[i];
          }
          d[j] = 0;
        }
      }
      i = j;
    }
    for(int i = end + 1; i < n; i++)
      a[i] = a[i - 1] + d[end];
    for(int i = start - 1; i >= 0; i--) {
      a[i] = a[i + 1] - d[start];
    }
    for(int i = 0; i < n && a[i] <= 0; i++)
      a[i] = 1;
    for(int i = n - 1; i >= 0 && a[i] <= 0; i--)
      a[i] = 1;
    int ans = 0;
    for(int i = 0, j = 0; i < n;) {
      j = i + 1;
      ans++;
      long v = 0;
      if(i < n - 1)
        v = a[j] - a[i];
      for(; j < n; j++) {
        if(a[j] - a[j - 1] != v) break;
      }
      i = j;
    }
    return ans;
  } 

  void solve() throws IOException {
    int n = nextInt();
    long[] a = new long[n];
    for(int i = 0; i < n; i++) {
      a[i] = nextLong();
    }
    long[] temp = new long[n];
    for(int i = 0; i < n; i++)
      temp[i] = a[i];
    int ans = Integer.MAX_VALUE;
    ans = Math.min(ans, get(temp));
    for(int i = n - 1; i >= 0; i--)
      temp[n - i - 1] = a[i];
    ans = Math.min(ans, get(temp));
    out.println(ans);
  }
}
