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

public class Main {
    public static void main(String[] args) throws Exception {
        Scanner sc = new Scanner();
        int left = sc.nextInt();
        int right = sc.nextInt();
        int size = right * 2 + 2;
        boolean[] isNotPrime = new boolean[size];
        for (int i = 2; i < size; i++) {
            if (isNotPrime[i]) {
                continue;
            }
            for (int j = 2; j * i < size; j++) {
                isNotPrime[j * i] = true;
            }
        }
        int count = 0;
        for (int i = Math.max(2, left); i <= right; i++)  {
            if (!isNotPrime[i]) {
                count++;
            }
        }
        for (int i = left * 2 + 1; i <= right * 2 + 1; i += 2) {
            if (!isNotPrime[i]) {
                count++;
            }
        }
        System.out.println(count);
    }
}
class Scanner {
    BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
    StringTokenizer st = new StringTokenizer("");
    
    public Scanner() throws Exception {
        
    }
    
    public int nextInt() throws Exception {
        return Integer.parseInt(next());
    }
    
    public long nextLong() throws Exception {
        return Long.parseLong(next());
    }
    
    public String nextLine() throws Exception {
        return br.readLine();
    }
    
    public String next() throws Exception {
        if (!st.hasMoreTokens()) {
            st = new StringTokenizer(br.readLine());
        }
        return st.nextToken();
    }
}