using System;
using System.Collections.Generic;
using System.Collections;
using System.Collections.Specialized;
using System.Linq;
using System.Text;
using System.IO;
using System.Reflection;
using static System.Math;
using System.Numerics;
static class Program{
	const int mod=(int)1e9+7;
	const double eps=1e-11;
	static void Main(){
		Sc sc=new Sc();
		var s=sc.Ia;
		if(s[0]==1){
			Console.WriteLine(0);
			return;
		}
		int ans=0;
		var li=new List<int[]>();
		var b=new bool[s[1]+1];
		for(int i = s[0];i<s[1];i++) {
			li.Add(new int[]{i-s[0],i+1-s[0],1});
			if(b[i]){continue;}
			for(int j = i*2;j<=s[1];j+=i) {
				li.Add(new int[]{i-s[0],j-s[0],0});
				b[j]=true;
			}
		}
		li.Sort((u,v)=>u[2]-v[2]);
		int n=s[1]-s[0]+1;
		Uf uf=new Uf(n);
		for(int i = 0;i<li.Count;i++) {
			if(!uf.Same(li[i][0],li[i][1])){
				uf.Union(li[i][0],li[i][1]);
				ans+=li[i][2];
			}
		}
		Console.WriteLine(ans);
	}
}
public class Uf{
	private int[] arr,hi;
	public int all;
	public Uf(int n){
		arr=new int[n];
		hi=new int[n];
		all=n;
		for(int i=0;i<n;i++){arr[i]=-1;}
	}
	public void Union(int a,int b){
		a=Root(a);
		b=Root(b);
		if(a!=b){
			if(hi[a]<hi[b]){a^=b;b^=a;a^=b;}
			arr[a]+=arr[b];
			hi[a]=Max(hi[a],hi[b]+1);
			arr[b]=a;
			all--;
		}
	}
	public bool Same(int a,int b){return Root(a)==Root(b);}
	public int Size(int a){return -arr[Root(a)];}
	public int Root(int a){return arr[a]<0?a:arr[a]=Root(arr[a]);}
}
public class Sc{
	public int I{get{return int.Parse(Console.ReadLine());}}
	public long L{get{return long.Parse(Console.ReadLine());}}
	public double D{get{return double.Parse(Console.ReadLine());}}
	public string S{get{return Console.ReadLine();}}
	public int[] Ia{get{return Array.ConvertAll(Console.ReadLine().Split(),int.Parse);}}
	public long[] La{get{return Array.ConvertAll(Console.ReadLine().Split(),long.Parse);}}
	public double[] Da{get{return Array.ConvertAll(Console.ReadLine().Split(),double.Parse);}}
	public string[] Sa{get{return Console.ReadLine().Split();}}
	public object[] Oa{get{return Console.ReadLine().Split();}}
	public int[] Ia2{get{return Array.ConvertAll(("0 "+Console.ReadLine()+" 0").Split(),int.Parse);}}
	public int[] Ia3(int a){return Array.ConvertAll((a.ToString()+" "+Console.ReadLine()).Split(),int.Parse);}
	public int[] Ia3(bool a,int b,bool c,int d){return Array.ConvertAll(((a?b.ToString()+" ":"")+Console.ReadLine()+(c?" "+d.ToString():"")).Split(),int.Parse);}
	public long[] La2{get{return Array.ConvertAll(("0 "+Console.ReadLine()+" 0").Split(),long.Parse);}}
	public long[] La3(int a){return Array.ConvertAll((a.ToString()+" "+Console.ReadLine()).Split(),long.Parse);}
	public long[] La3(bool a,int b,bool c,int d){return Array.ConvertAll(((a?b.ToString()+" ":"")+Console.ReadLine()+(c?" "+d.ToString():"")).Split(),long.Parse);}
	public double[] Da2{get{return Array.ConvertAll(("0 "+Console.ReadLine()+" 0").Split(),double.Parse);}}
	public double[] Da3(int a){return Array.ConvertAll((a.ToString()+" "+Console.ReadLine()).Split(),double.Parse);}
	public double[] Da3(bool a,int b,bool c,int d){return Array.ConvertAll(((a?b.ToString()+" ":"")+Console.ReadLine()+(c?" "+d.ToString():"")).Split(),double.Parse);}
	public T[] Arr<T>(int n,Func<T> f){var a=new T[n];for(int i=0;i<n;i++){a[i]=f();}return a;}
	public T[] Arr<T>(int n,Func<int,T> f){var a=new T[n];for(int i=0;i<n;i++){a[i]=f(i);}return a;}
	public T[] Arr<T>(int n,Func<string[],T> f){var a=new T[n];for(int i=0;i<n;i++){a[i]=f(Console.ReadLine().Split());}return a;}
	public T[] Arr<T>(int n,Func<int,string[],T> f){var a=new T[n];for(int i=0;i<n;i++){a[i]=f(i,Console.ReadLine().Split());}return a;}
}