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.BufferedReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.util.StringTokenizer;

public class pSize 
{

	public static void main(String[]args)throws Throwable
	{
		Scanner sc = new Scanner(System.in);
		int L = 0;
		int n = sc.nextInt();
		int arr[] = new int[n];
		for(int i = 0 ; i < n ; ++i)
			arr[i] = sc.nextInt();
		int ptr = 0;
		int ans = 0;
		while(ptr < n)
		{
		
			++ans;
			L = ptr;
			while(ptr < n && arr[ptr] == -1 )
				++ptr;
			if(ptr == n)
				break;
			int a1 = arr[ptr];
			int a1_pos = ptr;
			++ptr;
			while(ptr < n && arr[ptr] == -1)
				++ptr;
			if(ptr == n)
				break;
			long a2 = arr[ptr];
			int a2_pos = ptr;
			long step = (a2 - a1) / (a2_pos - a1_pos);
			
			if((a2 - a1) % (a2_pos - a1_pos) != 0)
			{
				ptr = a2_pos;
				continue;
			}
			if(step > 0 &&  a1 - 1L*step * (a1_pos - L) <= 0)
			{
				ptr = a2_pos;
				continue;
			}
			ptr = a2_pos + 1;
			long lst = a2;
			while(ptr < n)
			{
				long must = lst + step;
				if(must <= 0 || (arr[ptr] != -1 && must != arr[ptr]))
				{
					break;
				}
				lst += step;
				++ptr;
			}
			

		}
		System.out.println(ans);
	}

	static class Scanner
	{
		BufferedReader br;
		StringTokenizer st;


		Scanner(InputStream s)  { br = new BufferedReader(new InputStreamReader(s)); }

		String next() throws IOException
		{
			while(st == null || !st.hasMoreTokens())
				st = new StringTokenizer(br.readLine());
			return st.nextToken();

		}

		int nextInt() throws NumberFormatException, IOException { return Integer.parseInt(next()); }
		long nextLong() throws NumberFormatException, IOException { return Long.parseLong(next()); }


	}

}
