/**
 *    author:  ytsmash
 *    created: 27.08.2021 22:51:39
 **/

#include <bits/stdc++.h>
using namespace std;

typedef long long ll;
#define rep(i, n) for (int i = 0; i < (n); i++)
#define all(x) x.begin(), x.end()
template <class T>
bool chmax(T& a, const T& b) {
    if (a < b) {
        a = b;
        return 1;
    }
    return 0;
}
template <class T>
bool chmin(T& a, const T& b) {
    if (b < a) {
        a = b;
        return 1;
    }
    return 0;
}
const long double EPS = 1e-10;
const long long INF = 1e18;
const long double PI = acos(-1.0L);
using P = pair<int, int>;

bool isPrime(int x) {
    if (x < 2) return false;
    for (int i = 2; i * i <= x; i++) {
        if (x % i == 0) return false;
    }
    return true;
}

int main() {
    int L, R;
    cin >> L >> R;
    int ans = 0;
    for (int i = L; i <= R; i++) {
        if (isPrime(i)) ans++;
        if (isPrime(2 * i + 1) && i != R) ans++;
    }
    cout << ans << "\n";
    return 0;
}