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.StreamTokenizer;
import java.io.InputStreamReader;
import java.io.IOException;
import java.util.Arrays;
import java.io.BufferedReader;
import java.io.OutputStream;
import java.io.PrintWriter;
import java.io.Reader;
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;
		TokenizerReader in = new TokenizerReader(inputStream);
		PrintWriter out = new PrintWriter(outputStream);
		TaskD solver = new TaskD();
		solver.solve(1, in, out);
		out.close();
	}
}

class TaskD {
    public void solve(@SuppressWarnings("UnusedParameters") int testNumber, TokenizerReader in, PrintWriter out) {
        int n = in.nextInt();
        long[] a = new long[n];
        int prevKnown = -1;
        int[] nextKnown = new int[n];
        Arrays.fill(nextKnown, -1);

        for (int i = 0; i < n; ++i) {
            a[i] = in.nextInt();
            if (a[i] != -1) {
                for (int j = Math.max(0, prevKnown); j < i; ++j)
                    nextKnown[j] = i;
                prevKnown = i;
            }
        }

        int[] L = new int[n], R = new int[n];
        int i = 0;
        while (i < n && i >= 0) {
            if (a[i] == -1) {
                ++i;
                continue;
            }
            L[i] = i;
            R[i] = nextKnown[i];
            if (R[i] == -1) {
                L[i] = -1;
                break;
            }
            if ((a[R[i]] - a[i]) % (R[i] - i) != 0) {
                L[i] = R[i] = -1;
                ++i;
                continue;
            }
            long t = (a[R[i]] - a[i]) / (R[i] - i);
            while (L[i] > 0) {
                int j = L[i] - 1;
                long next = a[i] - t * (i - j);
                if (a[j] > 0 && a[j] != next)
                    break;
                if (next <= 0)
                    break;
                --L[i];
            }
            while (R[i] < n-1) {
                int j = R[i] + 1;
                long next = a[i] - t * (i - j);
                if (a[j] > 0 && a[j] != next)
                    break;
                if (next <= 0)
                    break;
                ++R[i];
            }
            int l = L[i], r = R[i];
            while (i >= 0 && nextKnown[i] >= 0 && nextKnown[i] <= r) {
                L[i] = l;
                R[i] = r;
                i = nextKnown[i];
            }
        }

        int[] dp = new int[n+1];
        dp[n] = 0;
        for (i = n-1; i >= 0; --i) {
            int u = nextKnown[i];
            if (u == -1) {
                dp[i] = 1;
                continue;
            }
            int v = nextKnown[u];
            if (v == -1) {
                dp[i] = 1;
                continue;
            }
            dp[i] = 1 + dp[v];
            int l = L[u], r = R[u];
            if (r >= v && l <= i) {
                int ch = 1;
                if (r+1 < n)
                    ch += dp[r+1];
                dp[i] = Math.min(dp[i], ch);
            }
        }

        out.println(dp[0]);
    }
}

class TokenizerReader extends StreamTokenizer {
    public TokenizerReader(InputStream in) {
        super(new BufferedReader(new InputStreamReader(in)));
        defaultConfig();
    }

    public String nextString() {
        try {
            nextToken();
        } catch (IOException e) {
            throw new RuntimeException("nextString: exception in line " + lineno(), e);
        }
        return sval;
    }

    public int nextInt() {
        return Integer.parseInt(nextString());
    }

    public void defaultConfig() {
        resetSyntax();
        wordChars(33, 126);
        whitespaceChars(0, ' ');
    }

}

