import java.io.*;
import java.util.*;

public class YaF {

    final static String file = "taxi";

    public static void solve(Input in, PrintWriter out) throws IOException {
        long len = in.nextLong();
        long d = in.nextLong();
        int n = in.nextInt();
        long[] xs = new long[n];
        for (int i = 0; i < n; ++i) {
            xs[i] = in.nextLong();
        }
        Arrays.sort(xs);
        int reserved = 0;
        while (reserved < n && xs[reserved] < len - d) {
            ++reserved;
        }
        if (reserved == n) {
            out.println(0);
            return;
        }
        if (xs[n - 1] >= d + len) {
            out.println(1);
            return;
        }
        long x = 0;
        int ans = 0;
        for (int i = n - 1; i >= 0; --i) {
            if (i == reserved) {
                continue;
            }
            if (xs[i] < d - x) {
                out.println(0);
                return;
            }
            ans++;
            x += xs[i] - (d - x);
            if (x >= len) {
                out.println(ans);
                return;
            }
            if (x + xs[reserved] - (d - x) >= len) {
                out.println(ans + 1);
                return;
            }
        }
        out.println(0);
    }

    public static void main(String[] args) throws IOException {
        PrintWriter out = new PrintWriter(file + ".out");
        solve(new Input(new BufferedReader(new FileReader(file + ".in"))), out);
        out.close();
    }

    static class Input {
        BufferedReader in;
        StringTokenizer st;

        public Input(BufferedReader in) {
            this.in = in;
            eat("");
        }

        public Input(String s) {
            this.in = new BufferedReader(new StringReader(s));
        }

        public void eat(String str) {
            st = new StringTokenizer(str);
        }

        public String next() throws IOException {
            while (!st.hasMoreTokens()) {
                String line = in.readLine();
                if (line == null) {
                    return null;
                }
                eat(line);
            }
            return st.nextToken();
        }

        public int nextInt() throws IOException {
            return Integer.parseInt(next());
        }

        public long nextLong() throws IOException {
            return Long.parseLong(next());
        }

        public double nextDouble() throws IOException {
            return Double.parseDouble(next());
        }
    }
}