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();
        UnionFindTree uft = new UnionFindTree(right + 1);
        boolean[] used = new boolean[right + 1];
        for (int i = left; i <= right; i++) {
            if (used[i]) {
                continue;
            }
            for (int j = 2; j * i <= right; j++) {
                uft.unite(i, j * i);
                used[j * i] = true;
            }
        }
        int ans = 0;
        for (int i = left; i < right; i++) {
            if (!uft.same(i, i + 1)) {
                ans++;
                uft.unite(i, i + 1);
            }
        }
        System.out.println(ans);
    }
    
    static class UnionFindTree {
        int[] parents;
        
        public UnionFindTree(int size) {
            parents = new int[size];
            for (int i = 0; i < size; i++) {
                parents[i] = i;
            }
        }
        
        public int find(int x) {
            if (x == parents[x]) {
                return x;
            } else {
                return parents[x] = find(parents[x]);
            }
        }
        
        public boolean same(int x, int y) {
            return find(x) == find(y);
        }
        
        public void unite(int x, int y) {
            if (!same(x, y)) {
                parents[find(y)] = find(x);
            }
        }
    }
}
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 double nextDouble() throws Exception {
        return Double.parseDouble(next());
    }
    
    public String next() throws Exception {
        if (!st.hasMoreTokens()) {
            st = new StringTokenizer(br.readLine());
        }
        return st.nextToken();
    }
}