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.InputStreamReader;

public class X416D {
    public static void main(String[] args) throws IOException {
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        String[] temp = br.readLine().split(" ");
        int n = Integer.parseInt(temp[0]);
        temp = br.readLine().split(" ");
        int[] a = new int[n];
        for (int i = 0; i < n; i++) {
            a[i] = Integer.parseInt(temp[i]);
        }

        int i = -1;
        int[] p1 = null;
        int[] p2 = null;
        int startI = 0;
        Integer d = null;
        Integer prev = null;
        int total = 0;
        while (++i < n) {
            if (d == null && a[i] == -1) continue;
            if (p1 == null) {
                p1 = new int[]{i, a[i]};
            } else if (p2 == null) {
                p2 = new int[]{i, a[i]};
                d = isValid(startI, p1, p2);
                if (d == null) {
                    total++;
                    p1 = p2;
                    p2 = null;
                    startI = i;
                } else {
                    prev = a[i];
                }
            } else {
                int curr = prev + d;
                if (curr <= 0 || (a[i] != -1 && a[i] != curr)) {
                    total++;
                    if (a[i] != -1) {
                        p1 = new int[]{i, a[i]};
                    } else {
                        p1 = null;
                    }
                    p2 = null;
                    startI = i;
                    d = null;
                    prev = null;
                } else {
                    prev = curr;
                }
            }
        }

        System.out.println(total + 1);
    }

    private static Integer isValid(int startI, int[] p1, int[] p2) {
        int di = p2[0] - p1[0];
        int dv = p2[1] - p1[1];
        double x = (double) dv / (double) di;
        if (x != (int) x) {
            return null;
        }
        int dF = p1[0] - startI;
        int first = p1[1] - dF * (int) x;
        if (first <= 0) return null;
        if (x == 0 || Math.abs(x) >= di) {
            return (int) x;
        } else {
            return null;
        }
    }
}
