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.*;
import java.io.*;
import java.math.*;
public class D {
    public static BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
    public static StringTokenizer st = null;
    public static String nx() throws Exception {
        for (;st == null || !st.hasMoreTokens();){
            String line = in.readLine();
            if (line == null) return line;
            st = new StringTokenizer(line);
        }
        return st.nextToken();
    }
    public static int ni () throws Exception {return Integer.parseInt(nx());}
    public static PrintWriter out = new PrintWriter(System.out);
    public static void px(Object...objects) {
        System.out.println(Arrays.deepToString(objects));
    }
    public static void main(String[] args) throws Exception {
        s();
        out.flush();
    }
    public static void s() throws Exception {
        int n1 = ni();
        int[] l1 = new int[n1];
        TreeSet<Integer> existingIndex = new TreeSet<>();
        for (int i = 0 ; i  < n1; i++){
            l1[i] = ni();
            if (l1[i] >= 1){
                existingIndex.add(i);
            }
        }
        int front = -1;
        int ans = 0;
        if (existingIndex.size() == 0){
            ans++;
        }
        for (;existingIndex.size() > 0;){
//          px(existingIndex);
            if (existingIndex.size() >= 2){
                int first = existingIndex.first();
                int second = existingIndex.higher(first);
                if ( (l1[second] - l1[first]) % (second - first) == 0){
//                  px("double");
                    int diff = (l1[second] - l1[first]) / (second - first);
                    int backDiff = -diff;
                    {
                        int curVal = l1[first];
                        for (int ptr = first - 1; ptr > front; ptr--){
                            curVal += backDiff;
                            if (curVal <= 0){
                                ans++;
                                break;
                            }
                            l1[ptr] =curVal;
                        }
                    }
                    existingIndex.pollFirst();
                    existingIndex.pollFirst();
                    {
                        front = second;
                        int curVal = l1[second];
                        for (int ptr = second + 1; ptr < l1.length; ptr++){
                            curVal += diff;
                            if (curVal <= 0){
                                break;
                            }
                            if (l1[ptr] != -1 && l1[ptr] != curVal){
                                break;
                            }
                            if (l1[ptr] != -1) existingIndex.pollFirst();
                            l1[ptr] = curVal;
                            front = ptr;
                            
                        }
                    }
                    ans++;
                }
                else {
//                  px("single");
                    ans += 1;
                    existingIndex.pollFirst();
                    front = first;
                }
            }
            else {
//              px("last");
                ans++;
                int last = existingIndex.pollFirst();
                int before = 0;
                for (int ptr = last -1; ptr > front; ptr--){
                    if (l1[ptr] == -1){
                        before++;
                    }
                }
                int after = l1.length - last - 1;
                if (before > l1[last] - 1 && after > l1[last] - 1){
                    ans++;
                }
            }
        }
//      px(ans, l1);
//      for (int i = 0; i < l1.length; i++){
//          if (l1[i] == -1){
//              if (i == 0){
//                  l1[i] = 1;
//              }
//              else {
//                  l1[i] = l1[i - 1] + 1;
//              }
//          }
//      }
//      px(l1);
        out.println(ans);
    }
}
