#include<bits/stdc++.h>
using namespace std;
bool sushu[99999999];
long long n;
int main()
{
    int m,x,max=0;
    cin>>m>>n;
    for(int i=m;i<=n;++i)
    {
        if(i%2==0)
        sushu[i]=false;
        else
        sushu[i]=true;
    }
    sushu[1]=false;
    for(int i=m;i*i<=n;++i)
    {
        x=2;
        if(sushu[i]==true)
        while(i*x<=n)
        {
            sushu[i*x]=false;
            ++x;
        }    
    }
    sushu[2]=true;
    for(int i=m;i<=n;++i)
    if(sushu[i]==true)
    max++;
    cout<<max;
}