#include <bits/stdc++.h>

#define MAX_LR (1000000)

using namespace std;

typedef long long ll;

int main() {
    ll l, r;
    bool is_prime_memo[MAX_LR + MAX_LR + 1 + 1];
    int ans = 0;

    scanf("%lld %lld", &l, &r);

    for (int i = 1; i <= MAX_LR + MAX_LR + 1; i++) {
        is_prime_memo[i] = true;
    }
    is_prime_memo[1] = false;
    for (int i = 2; i <= MAX_LR + MAX_LR + 1; i++) {
        for (int j = i + i; j <= MAX_LR + MAX_LR + 1; j+=i) {
            is_prime_memo[j] = false;
        }
    }

    for (ll i = l; i <= r; i++) {
        if (is_prime_memo[i]) {
            ans++;
        }
        if (i+1 <= r && is_prime_memo[i+i+1]) {
            ans++;
        }
    }

    printf("%d\n", ans);
    
    return 0;
}