#include <bits/stdc++.h>
#define ll long long
using namespace std;
int a,b;
bool snt(long long x)
{
    if(x<2)return false;
    if(x<4)return true;
    if(x%2==0||x%3==0)return false;
    for(int i=5; i*i<=x; i=i+6)
    {
        if(x%i==0||x%(i+2)==0)
        {
            return false ;
        }
    }
    return true;
}
signed main()
{
    cin>>a>>b;
    int dem=0;
    for(int i=a;i<=b;i++)
    {
        if(snt(i)) dem++; // lười sàng vl
    }
    cout<<dem;
    return 0;
}