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 D {

    public static void main(String[] args) {
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        try {
            br.readLine();
            String s = br.readLine();
            String as[] = s.split(" ");

            int increment = Integer.MAX_VALUE; // if 0 then equal
            int decrement = Integer.MAX_VALUE;
            int last_i_pos = -1; //last i of positive p_i
            int ans = 0;
            int p[] = new int[as.length];
            boolean front_open = false;
            for (int i = 0; i < as.length; ++i) {
                p[i] = Integer.parseInt(as[i]);
                if (p[i] == -1) {
                    if (increment == Integer.MAX_VALUE && decrement == Integer.MAX_VALUE) {
                        // any inc or dec but now -1
                        // continue
                    } else if (increment != Integer.MAX_VALUE) {
                        // we have specific inc but now -1
                        // continue
                    } else {
                        // we have specific dec but now -1
                        // continue
                    }
                } else if (increment == Integer.MAX_VALUE && decrement == Integer.MAX_VALUE) {
                    // any inc or dec but now non -1
                    if (last_i_pos == -1) {
                        // we still cant decide, continue but note that
                        // the most front sequence is still open
                        front_open = true;
                    } else {
                        // there was non -1 with undecided inc/dec followed by a sequence of -1 then p[i]
                        int diff = p[i] - p[last_i_pos];
                        if (Math.abs(diff) % (i - last_i_pos) == 0) {
                            // we have specific diff, but we can continue
                            if (diff >= 0) {
                                increment = diff / (i - last_i_pos);
                                if (front_open) {
                                    if (p[last_i_pos] - last_i_pos * increment > 0) {
                                        // the front is open and the distance is
                                        // enough to provide this sequence
                                    } else {
                                        // the distance is not enough
                                        ans++;
                                        front_open = false;
                                    }
                                }
                            } else {
                                decrement = diff / (i - last_i_pos);
                            }
                        } else {
                            // we have to stop the sequence and start a new one
                            ans++;
                            // cant decide the inc/dec of the new sequence 
                        }
                    }
                } else if (increment != Integer.MAX_VALUE) {
                    // we have specific inc but now non -1
                    if (p[i] == (i - last_i_pos) * increment + p[last_i_pos]) {
                        // p[i] match the inc, we can continue
                    } else {
                        // we have to stop the sequence and start a new one
                        ans++;
                        // cant decide the inc/dec of the new sequence 
                        increment = Integer.MAX_VALUE;
                    }
                } else // we have specific dec but now non -1
                 if (p[i] == (i - last_i_pos) * decrement + p[last_i_pos]) {
                        // p[i] match the inc, we can continue
                    } else {
                        // we have to stop the sequence and start a new one
                        ans++;
                        // cant decide the inc/dec of the new sequence 
                        decrement = Integer.MAX_VALUE;
                    }
                if (p[i] != -1) {
                    last_i_pos = i;
                }
                //System.out.println(increment + " " + decrement);
            }
            // close last sequence
            ans += 1;
            if (decrement != Integer.MAX_VALUE && last_i_pos != p.length - 1) {
                // check for closing the rear
                if (p[last_i_pos] + decrement * (p.length - last_i_pos- 1) <= 0) {
                    // the last -1 sequence does not provide enough for one sequence
                    ans++;
                }
            }
            System.out.println(ans);
        } catch (IOException ex) {
        }
    }
}
