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.Scanner;

public class D {

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		long[] a = new long[n];
		for(int i=0;i<n;i++) {
			a[i] = sc.nextInt();
		}
		int i = 0;
		int beginindex = -1;
		int leftindex = -1;
		long leftnum = 0;
		int ans = 0;
		while(i < n) {
			if (beginindex == -1) {
				beginindex = i;
			}
			if (a[i] != -1) {
				if (leftindex == -1) {
					leftindex = i;
					leftnum = a[i];
				}else{
					int rightindex = i;
					long rightnum = a[i];
					if ((rightnum-leftnum)%(rightindex-leftindex)==0) {
						long d = (rightnum-leftnum)/(rightindex-leftindex);
						long beginnum = leftnum - d * (leftindex-beginindex);
						if (beginnum <= 0) {
							ans++;
						}
						while(i+1 < n) {
							long num = rightnum - d * (rightindex - (i+1));
							if (num > 0 && (a[i+1] == -1 || a[i+1] == num)) {
								i++;
							}else{
								break;
							}
						}
						ans++;
						beginindex = -1;
						leftindex = -1;
					}else{
						ans++;
						beginindex = i;
						leftindex = i;
						leftnum = a[i];
					}
				}
			}
			if (i==n-1 && beginindex!=-1) {
				ans++;
			}
			i++;
		}
		System.out.println(ans);
	}

}
