#include<bits/stdc++.h>
using namespace std;
int qq(int q){
    int o=1;
    if(q>1){
        for(int i=2;i<q;i++){
            if(q%i==0){
                o=0;
                break;
            }
        }
    }
    else{
        o=0;
    }
    return o;
}
int main()
{
    int i,n,m,s=0;
    cin>>m>>n;
    for(i=m;i<=n;i++){
        if(qq(i)==1)
            s++;
    }
    cout<<s;
return 0;
}
/**************************************************************
	Problem: 1063
	User: lijinkai
	Language: C++
	Result: Accepted
	Time:126 ms
	Memory:2072 kb
****************************************************************/