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.InputStreamReader;


public class Q4 {

    /**
     * @param args
     */
    public static void main(String[] args) throws Exception {
        // TODO Auto-generated method stub
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        int total= Integer.valueOf(br.readLine());
        String all[] = br.readLine().split(" ");
        int num[] = new int[total];
        for(int i = 0; i < total; i++) {
            num[i] = Integer.valueOf(all[i]);
        }
        
        int i = 0;
        int before = 0;
        int after = 0;
        int first = -1;
        int numProg = 0;
        while(i < total) {
            if(first == -1) {
                while(i < total && num[i] == -1) {
                    before++;
                    i++;
                }
                if(i == total) {
                    numProg++;
                    break;
                }
                first = i;
                i++;
            } else {
                while(i < total && num[i] == -1) {
                    after++;
                    i++;
                }
                if(i == total) {
                    numProg++;
                    first = -1;
                    break;
                }
                if(Math.abs(num[i] - num[first]) % (after + 1) == 0) {
                    int step = (num[i] - num[first])/(after + 1);
                    if(num[first] - step * before <= 0) {
                        numProg++;
                    }
                    i++;
                    while(i < total && (num[i-1] + step > 0) && (num[i] == -1 || num[i] - num[i-1] == step)) {
                        if(num[i] == -1) {
                            num[i] = num[i-1] + step;
                        }
                        i++;
                    }
                    if(i == total) {
                        numProg++;
                        first = -1;
                        break;
                    }
                    numProg++;
                    first = -1;
                    //i++;
                    before = 0;
                    after = 0;
                } else {
                    numProg++;
                    first = i;
                    i++;
                    before = 0;
                    after = 0;
                }
            }
        }
        if(first != -1) {
            numProg++;
        }
        System.out.print(numProg);
    }

}
