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.File;
import java.io.FileInputStream;
import java.io.InputStream;
import java.util.ArrayList;
import java.util.List;
import java.util.Scanner;

/**
 * Codejam 2014
 * User: maurya
 * Date: 14/4/14
 * Time: 1:25 AM
 */
public class Main {
    public static void main(String args[]) throws Exception {
        InputStream is = System.in;
        if (args != null && args.length > 0 && args[0] != null) {
            is = new FileInputStream(new File(args[0]));
        }
        Scanner in = new Scanner(is);
        int n = in.nextInt();
        List<Integer> a = new ArrayList<Integer>();
        for (int i = 0; i < n; i++) {
            if (i == 0) {

            }
            if (i < n)
            a.add(in.nextInt());
        }
        int ii = 0;
        while(ii < a.size() && a.get(ii) == -1){
            a.remove(ii);
        }
        ii = a.size() -1;
        while (ii >= 0 && a.get(ii) == -1) {
            a.remove(ii);
            ii--;
        }
        int diff = 1<<30;
        int prev = -1;int gap = 0;
        int res = 1;
        for (int  i = 0; i < a.size(); i++) {
            if (diff == 1<<30 && a.get(i) == -1) {
                if (prev != -1) {
                    gap++;
                }
            }
            else if (diff == 1<<30 && a.get(i) != -1) {
                if (gap != 0 && prev != -1 && (prev - a.get(i))%gap == 0) {
                    diff = (a.get(i) - prev)/gap;
                } else if (gap == 0 && prev != -1){
                    diff = (a.get(i) - prev);
                }
                prev = a.get(i);
                gap = 0;
            }
            else if (diff != 1<<30 && a.get(i) == -1) {
                gap++;
            }
            else if (diff != 1<<30 && a.get(i) != -1) {
                if (gap != 0 && (a.get(i) - prev)%gap == 0 && (a.get(i) - prev)/gap == diff) {
                } else if (gap == 0 && a.get(i) == prev + diff){

                } else {
                    res++;
                    diff = 1<<30;
                }
                prev = a.get(i);

                gap = 0;
            }
        }
        System.out.println(res);
    }
}

