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;
import java.util.Arrays;

public class Div241A {
    public static void main(String[] args) throws NumberFormatException,
            IOException {
        BufferedReader bf = new BufferedReader(new InputStreamReader(System.in));
        int n = Integer.parseInt(bf.readLine());
        String l[] = bf.readLine().split(" ");
        int nums[] = new int[n];
        int currNInd = -1;
        int lastNInd = -1;
        for (int i = 0; i < n; i++) {
            nums[i] = Integer.parseInt(l[i]);
            if (nums[i] != -1 && currNInd == -1)
                currNInd = i;
            if (nums[i] != -1)
                lastNInd = i;
        }
        if (currNInd == -1 || n == 1) {
            System.out.println(1);
            return;
        }
        int diff = 0, prev = 0, nextInd;
        boolean d = false;
        boolean changed[] = new boolean[n];
        for (int i = currNInd + 1; i < nums.length; i++) {
            if (nums[i] == -1) {
                if (d && nums[i] + diff > 0) {
                    nums[i] = prev + diff;
                    changed[i] = true;
                    prev = nums[i];
                } else {
                    d = false;
                }
            } else {
                if (!changed[i - 1] && nums[i - 1] != -1) {
                    diff = nums[i] - nums[i - 1];
                    prev = nums[i];
                    d = true;
                } else {
                    d = false;
                }
            }
        }
        d = false;
        for (int i = lastNInd - 1; i >= 0; i--) {
            if (nums[i] == -1) {
                if (d && prev + diff > 0) {
                    nums[i] = prev + diff;
                    changed[i] = true;
                    prev = nums[i];
                } else {
                    d = false;
                }
            } else {
                if (!changed[i + 1] && nums[i + 1] != -1) {
                    diff = nums[i] - nums[i + 1];
                    prev = nums[i];
                    d = true;
                } else {
                    d = false;
                }
            }
        }
        int nextN[] = new int[n];
        Arrays.fill(nextN, -1);
        int firstInd = currNInd;
        for (int i = currNInd + 1; i < nums.length; i++) {
            if (nums[i] != -1) {
                nextN[currNInd] = i;
                currNInd = i;
            }
        }
        currNInd = firstInd;
        while (true) {
            nextInd = nextN[firstInd];
            if (nextInd == -1) {
                if (firstInd == 0) {
                    for (int i = firstInd + 1; i < n; i++) {
                        nums[i] = nums[i - 1] + 1;
                    }
                } else {
                    if (nums[firstInd - 1] == -1) {
                        int right = n - firstInd - 1;
                        int left = 0;
                        int c = left - 1;
                        while (c >= 0 && nums[c] == -1) {
                            left++;
                            c--;
                        }
                        diff = 1;
                        if (left > right) {
                            diff = -1;
                        }
                        for (int i = firstInd + 1; i < n; i++) {
                            if (nums[i - 1] + diff <= 0) {
                                diff = 1;
                            }
                            nums[i] = nums[i - 1] + diff;
                        }
                        diff = 1;
                        if (left > right) {
                            diff = -1;
                        }
                        c = Math.max(c, 0);
                        for (int i = firstInd - 1; i >= c; i--) {
                            if (nums[i + 1] + diff <= 0) {
                                diff = -1;
                            }
                            nums[i] = nums[i + 1] - diff;
                        }
                    } else {
                        diff = nums[firstInd] - nums[firstInd - 1];
                        for (int i = firstInd + 1; i < n; i++) {
                            nums[i] = nums[i - 1] + diff;
                            if (nums[i] <= 0) {
                                nums[i] = nums[i - 1] + 1;
                                diff = 1;
                            }
                        }
                    }
                }
                break;
            }
            diff = Math.abs(nums[firstInd] - nums[nextInd]);
            if (diff % (nextInd - firstInd) == 0) {
                diff /= (nextInd - firstInd);
                if (nums[firstInd] > nums[nextInd])
                    diff *= -1;
                for (int i = firstInd + 1; i < nextInd; i++) {
                    nums[i] = nums[i - 1] + diff;
                }
            } else {
                diff = 1;
                if (nums[nextInd] % (nextInd - firstInd + 1) == 0) {
                    diff = -(nums[nextInd] / (nextInd - firstInd + 1));
                }
                for (int i = nextInd - 1; i > firstInd; i--) {
                    nums[i] = nums[i + 1] + diff;
                }
            }
            firstInd = nextInd;
        }
        firstInd = currNInd;
        if (firstInd > 0 && nums[firstInd - 1] == -1) {
            diff = 1;
            if (firstInd + 1 < n) {
                diff = (nums[firstInd] - nums[firstInd + 1]);
            }
            for (int i = firstInd - 1; i >= 0; i--) {
                nums[i] = nums[i + 1] + diff;
                if (nums[i] <= 0) {
                    nums[i] = nums[i + 1] + 1;
                    diff = 1;
                }
            }
        }
        //System.out.println(Arrays.toString(nums));
        System.out.println(countMin(nums));
    }

    private static int countMin(int[] nums) {
        if (nums.length == 1)
            return 1;
        int index = 1;
        for (int i = 1; i < nums.length - 1; i++) {
            if (nums[i] - nums[i + 1] != nums[i - 1] - nums[i]) {
                index++;
                i++;
            }
        }
        return index;
    }
}
