s

#include <bits/stdc++.h>
using namespace std;
int main() {
    int a,b,count=0;
    cin>>a>>b;
    if (a>b)swap(a,b);
    if (a<2)a=2;
    if (b<2) {
        cout<<0<< endl;
        return 0;
    }
    vector<bool> isPrime(b + 1, true);
    isPrime[0]=isPrime[1] =false;
    for (int i = 2; i * i <= b; ++i)
        if (isPrime[i])
            for (int j = i * i; j <= b; j += i)
                isPrime[j] = false;
        for (int i = a; i <= b; ++i)
        if (isPrime[i]) count++;
    
    cout << count << endl;
    return 0;
}