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 Population
{
    Scanner in;
    boolean submit = true;
    
    int i;
    
    public static void main(String args[]) throws Exception
    {
        new Population().run();
    }
    
    void run() throws Exception
    {
        if (!submit)
            in = new Scanner(new FileReader("testdata.txt"));
        else
            in = new Scanner(System.in);
        
        Integer progression = null;
        int answer = 0;
        
        // loads data
        int numNums = in.nextInt();
        int[] num = new int[numNums];
        for (int i = 0; i < numNums; i++)
            num[i] = in.nextInt();
        
        i = 1;
        while (i < numNums)
        {
            answer++;
            
            int skipped = 0;
            
            while (i < numNums && num[i-1] == -1)
            {
                i++;
                skipped++;
            }
            
            int last = i-1;
            int span = 1;
            
            while (i < numNums && num[i] == -1)
            {
                i++;
                span++;
            }
            
            if (i == numNums)
            {
                if (num[last] == 1)
                    answer++;
                break;
            }
            
            progression = (num[i] - num[last]) / span;
            
            if (num[last] == 1 && skipped > 0 && progression > 0)
            {
                last = i;
                i++;
                if (i >= numNums)
                    answer++;
                continue;
            }
            
            if (span > 1 && Math.abs(num[i] - num[last]) == 1)
            {
                answer++;
                last = i;
                i++;
                continue;
            }
            
            last = i;
            i++;
            
            while (i < numNums)
            {
                span = 1;
                while (i < numNums && num[i] == -1)
                {
                    i++;
                    span++;
                }
                
                if (i == numNums)
                {
                    if (num[last] + (progression * span) < 1)
                        answer++;
                    break;
                }
                
                if ((num[i] - num[last]) / span != progression)
                    break;
                
                last = i;
                i++;
            }
            
            if (i != numNums - 1)
                i++;
        }
        
        if (answer != 0)
            System.out.println(answer);
        else
            System.out.println(1);
    }
}