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

/*
 * To change this license header, choose License Headers in Project Properties.
 * To change this template file, choose Tools | Templates
 * and open the template in the editor.
 */

//package d;
import java.io.*;
import java.util.*;



/**
 *
 * @author Vlad
 */
public class D {

    /**
     * @param args the command line arguments
     * @throws java.io.IOException
     */
    public static void main(String[] args) throws IOException {
        InputReader in = new InputReader(System.in);
        PrintWriter out = new PrintWriter(System.out);
        
        int n = in.nextInt();
        List<Integer> list = new ArrayList<>();
        
        boolean fl = false;
        for (int i = 0; i < n; i++) {
            list.add(in.nextInt());
            if (list.get(i) != -1) fl = true;
            
        }
        
        if (list.size() == 1 || !fl) {
            out.println(1);
            out.close();
            return;
        }
        
        int kil = 0;
        int stan = 0;
        int delta = Integer.MAX_VALUE;
        int cur = 0;
        int prev = 0;
        int kilminus = 0;
        int last = Integer.MIN_VALUE;
        
        
        int start = 0;
        int ii = 0;
        while (list.get(ii) == -1) {
            ii++;
            kilminus++;
        }
        
        if (kilminus > 0 && list.get(ii) == 1) kil++;
        start = ii;
        kilminus = 0;
        
        
        for (int i = start; i < list.size(); i++) {
            switch(stan) {
                case 0: 
                    if (list.get(i) != -1) {
                        kilminus = 0;
                        stan = 1;
                        kil++;
                        prev = list.get(i);
                        last = prev;
                    }
                    break;
                    
                case 1: 
                    if (list.get(i) != -1) {
                        kilminus = 0;
                        delta = prev - list.get(i);
                        stan = 2;
                        last = list.get(i);
                    } else {
                        stan = 3;
                        kilminus++;
                    }
                    break;
                    
                case 2: 
                    if (list.get(i) != -1) {
                        kilminus = 0;
                        if (list.get(i-1) - list.get(i) != delta) {
                        stan = 1;    
                        kil++;
                        prev = list.get(i);
                        last = prev;
                        }
                    } else {
                        stan = 3;
                        kilminus++;
                    }
                    break;
                case 3:
                    if (list.get(i) != -1) {
                        int part = Math.abs((list.get(i) - last) / (kilminus+1));
                        //out.println(part + " " + last + " " + list.get(i) + " " + kilminus);
                        if (part * (kilminus+1) == Math.abs(list.get(i) - last) && (list.get(i) - last > 1)) {
                            stan = 2;
                            kilminus = 0;
                        } else {
                            kil++;
                            prev = list.get(i);
                            last = prev;
                            stan = 1;
                            kilminus = 0;
                        }
                        
                    } else {
                        kilminus++;
                    }
                    
                    break;
                    
                
            }
        }
        out.println(kil);
        out.close();
        // TODO code application logic here
    }
    
}


class InputReader {
    private final BufferedReader reader;
    private StringTokenizer tokenizer;
    
    InputReader(InputStream stream) {
        reader = new BufferedReader(new InputStreamReader(stream));
        tokenizer = new StringTokenizer("");
    }
    
    public String getLine() {
        try {
            return reader.readLine();
        } catch (IOException ex) {
            throw new RuntimeException(ex);
        }
    }
    
    public String next() throws IOException {
        while (!tokenizer.hasMoreTokens()) {
            tokenizer = new StringTokenizer(reader.readLine());
        }
        return tokenizer.nextToken();
    }
    
    public int nextInt() throws IOException {
        return Integer.parseInt(next());
    }
    
    public long nextLong() throws IOException {
        return Long.parseLong(next());
    }
    
    public double nextDouble () throws IOException {
        return Double.parseDouble(next());
    }
      
}