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.IOException;
import java.io.InputStream;
import java.io.PrintWriter;
import java.util.Arrays;
import java.util.InputMismatchException;

/**
 * Created by hama_du on 2014/09/21.
 */
public class ProblemD {

    public static void main(String[] args) {
        InputReader in = new InputReader(System.in);

        int n = in.nextInt();
        long[] a = new long[n];
        for (int i = 0; i < n; i++) {
            a[i] = in.nextLong();
        }

        int cnt = 0;
        for (int i = 0 ; i < n ;) {
            cnt++;
            int fr = i;
            int to = i-1;
            int nonem1 = 0;
            int ci = -1;
            long ca = 0;
            long cd = 0;
            while (true) {
                to++;
                if (to >= n) {
                    i = n;
                    break;
                }
                if (nonem1 <= 1) {
                    if (a[to] != -1) {
                        if (nonem1 == 0) {
                            ci = to;
                            ca = a[to];
                            cd = 0;
                            nonem1++;
                        } else if (nonem1 == 1) {
                            cd = (a[to] - a[ci]) / (to - ci);
                            boolean isok = Math.abs(a[to] - a[ci]) % (to - ci) == 0;
                            for (int f = fr ; f <= to ; f++) {
                                long expected = ca + cd * (f - ci);
                                if (expected >= 1 && (a[f] == -1 || a[f] == expected)) {
                                } else {
                                    isok = false;
                                    break;
                                }
                            }
                            if (!isok) {
                                i = to;
                                break;
                            }
                            nonem1 = 2;
                        }
                    }
                } else {
                    long expected = ca + cd * (to - ci);
                    if (expected >= 1 && (a[to] == -1 || a[to] == expected)) {
                        continue;
                    } else {
                        i = to;
                        break;
                    }
                }
            }
        }

        PrintWriter out = new PrintWriter(System.out);
        out.println(cnt);
        out.flush();
    }



    static int[][][] buildWeightedGraph(InputReader in, int n, int m) {
        int[][] edges = new int[m][];
        int[][][] graph = new int[n][][];
        int[] deg = new int[n];
        for (int i = 0 ; i < m ; i++) {
            int a = in.nextInt()-1;
            int b = in.nextInt()-1;
            int w = in.nextInt();
            deg[a]++;
            deg[b]++;
            edges[i] = new int[]{a, b, w};
        }
        for (int i = 0 ; i < n ; i++) {
            graph[i] = new int[deg[i]][2];
        }
        for (int i = 0 ; i < m ; i++) {
            int a = edges[i][0];
            int b = edges[i][1];
            int w = edges[i][2];
            graph[a][--deg[a]][0] = b;
            graph[b][--deg[b]][0] = a;
            graph[a][deg[a]][1] = w;
            graph[b][deg[b]][1] = w;
        }
        return graph;
    }

    public static void debug(Object... o) {
        System.err.println(Arrays.deepToString(o));
    }


    static class InputReader {
        private InputStream stream;
        private byte[] buf = new byte[1024];
        private int curChar;
        private int numChars;
        private SpaceCharFilter filter;

        public InputReader(InputStream stream) {
            this.stream = stream;
        }

        public int next() {
            if (numChars == -1)
                throw new InputMismatchException();
            if (curChar >= numChars) {
                curChar = 0;
                try {
                    numChars = stream.read(buf);
                } catch (IOException e) {
                    throw new InputMismatchException();
                }
                if (numChars <= 0)
                    return -1;
            }
            return buf[curChar++];
        }

        public int nextInt() {
            int c = next();
            while (isSpaceChar(c))
                c = next();
            int sgn = 1;
            if (c == '-') {
                sgn = -1;
                c = next();
            }
            int res = 0;
            do {
                if (c < '0' || c > '9')
                    throw new InputMismatchException();
                res *= 10;
                res += c - '0';
                c = next();
            } while (!isSpaceChar(c));
            return res * sgn;
        }

        public long nextLong() {
            int c = next();
            while (isSpaceChar(c))
                c = next();
            long sgn = 1;
            if (c == '-') {
                sgn = -1;
                c = next();
            }
            long res = 0;
            do {
                if (c < '0' || c > '9')
                    throw new InputMismatchException();
                res *= 10;
                res += c - '0';
                c = next();
            } while (!isSpaceChar(c));
            return res * sgn;
        }

        public boolean isSpaceChar(int c) {
            if (filter != null)
                return filter.isSpaceChar(c);
            return c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1;
        }

        public interface SpaceCharFilter {
            public boolean isSpaceChar(int ch);
        }
    }
}
