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.util.Arrays;
import java.util.InputMismatchException;
import java.io.OutputStream;
import java.io.PrintWriter;
import java.math.BigInteger;
import java.io.InputStream;

/**
 * Built using CHelper plug-in
 * Actual solution is at the top
 */
public class Main {
	public static void main(String[] args) {
		InputStream inputStream = System.in;
		OutputStream outputStream = System.out;
		InputReader in = new InputReader(inputStream);
		PrintWriter out = new PrintWriter(outputStream);
		TaskD solver = new TaskD();
		solver.solve(1, in, out);
		out.close();
	}
}

class TaskD {
    int a[];
    int dp[];
    public void solve(int testNumber, InputReader in, PrintWriter out) {
        int n = in.readInt();
        a = new int[n];
        for (int i = 0; i < n; i++) a[i] = in.readInt();
        dp = new int[n + 1];
        Arrays.fill(dp, -1);
        out.println(go(0));
    }
    int go(int pos) {
        if (pos == a.length) return 0;
        if (dp[pos] != -1) return dp[pos];
        int d = Integer.MIN_VALUE;

        int ans = Integer.MAX_VALUE;
        int index = -2;
        for (int i = pos; i < a.length; i++) {
            if (i < pos + 2) {
                ans = Math.min(ans, go(i + 1) + 1);
                if (i == pos + 1 && (a[i] != -1 && a[i - 1] != -1)) d= a[i] - a[i - 1];
            }
            else if (a[i] == -1) ans = Math.min(ans, go(i + 1) + 1);
            else {
                if (d == Integer.MIN_VALUE) {
                    if (index == i - 1) d = a[i] - a[index];
                    if (index != -2) {
                        if ((a[i] - a[index]) % (i - index) == 0) d = (a[i] - a[index]) / (i - index);
                        else break;
                    }
                }
                else {
                    if (a[i] != a[index] + (i - index) * d) break;
                }
                ans = Math.min(ans, go(i + 1) + 1);
            }
            if (a[i] >= 0) index = i;
        }
        return dp[pos] = ans;

    }
}

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

        public boolean isSpaceChar(int c) {
                if (filter != null)
                        return filter.isSpaceChar(c);
                return isWhitespace(c);
        }

        public static boolean isWhitespace(int c) {
                return c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1;
        }

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

