#include<iostream>
#include<cstdlib>
#include<cstdio>
#include<cmath>
#include<string>
#include<cstring>
#include<algorithm>
using namespace std;
const int MAX=50010;
int f[MAX];
bool b[MAX];
bool a[1000001];
void zyz()
{
	int i,j,k,t=0;
	bool p;
	memset(f,0,sizeof(f));
	memset(b,false,sizeof(b));
	t=1; f[1]=2; i=3; b[2]=true;
	while(i<=50000)
	{
	    k=int(sqrt(i));
		p=true;
		for(j=2; j<=k; j++)
		{
			if(i%j==0)
			{
			    p=false;
				break;	
			} 	
		}	
		if(p)
		{
		    t++;
			f[t]=i;
			b[i]=true;	
		}
		i+=2;
	}
}
int main()
{
	int l,r,ans,i,j,t,k;
	cin>>l>>r;
	zyz();
	ans=0;
	if(r<=50000)
	{
	    for(int i=1; i<=r; i++)
		{
		    if(b[i]) ans++;	
		}	
	}
	else
	{
	    if(l<=50000)
		{
		    for(int i=1; i<=50000; i++)
			{
				if(b[i]) ans++;	
			}	
			l=50001;
		}	
		k=int(sqrt(r));
		i=1;
		while(k>f[i])   i++;
		k=i;
		memset(a,true,sizeof(a));
		for(int i=1; i<=k; i++)
		{
		    for(int j=l/f[i]; j<=r/f[i]; j++)
		    {
			     if((f[i]*j<=r)&&(f[i]*j>=l))
				 {
				     a[f[i]*j-l]=false;		
				}	
			}
		}
		for(int i=0; i<=r-l; i++)
		{
		     if(a[i])  ans++;	
		}
	}
	cout<<ans<<endl;
	return 0;
}