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.util.*;
import static java.lang.System.out;

public class Main{
	
	//key is index in array, value is min for that index
	public static Map<Integer, Integer> map = new HashMap<Integer, Integer>();
	public static int[] a;
	
	public static void main(String[] args) {
		Scanner s = new Scanner(System.in);
		int n = s.nextInt();
		a = new int[n];
		for(int x=0;x<n;x++)
			a[x] = s.nextInt();
		
		out.println(recur(0));
	}
	
	public static int recur(int index) {
		if(index>=a.length-2)
			return 1;
		
		if(map.get(index)!=null)
			return map.get(index);
		
		if(a[index]==-1)
			return recur(index+1);
		
		int max = getMax(index);
			
		int min = a.length-index;
		for(int x=0;x<max;x++) {
			int temp = 1 + recur(index+1+x);
			if(temp<min)
				min = temp;
		}
		
		map.put(index,min);
		
		return min;
	}
	
	public static int getMax(int index) {
		int i = index+1;
		while(a[i]==-1)
			i++;
		int dif = (a[index]-a[i])/(i-index);
		
		int max = 2;
		int prev = a[index];
		boolean dud = false;
		for(int x=index+1;x<a.length-1 && !dud;x++)
			if(a[x]==-1) {
				max++;
				prev+= dif;
			} else if(prev - a[x]==dif) {
				max++;
				prev = a[x];
			} else
				dud = true;
			
		return max;
	}
}
			  	 	   	 	 		 	 	 			    	