#include <bits/stdc++.h>
#define rep(i, n) for (int i = 0; i < (n); ++i)

using std::cin;
using std::cout;
using std::set;
using std::vector;
using ll = long long;

// linear sieve
vector<int> ps, pf;
void sieve(int mx) {
    pf.resize(mx + 1);
    rep(i, mx + 1) pf[i] = i;
    for (int i = 2; i <= mx; ++i) {
        if (pf[i] == i) ps.push_back(i);
        for (int j = 0; j < ps.size() and ps[j] <= pf[i]; ++j) {
            int x = ps[j] * i;
            if (x > mx) break;
            pf[x] = ps[j];
        }
    }
}

int main() {
	const int MX = 2000005;
	sieve(MX);
	set<int> s;
	for (int p : ps) s.insert(p);
	
	ll l, r;
	cin >> l >> r;
	
	int ans = 0;
	for (int i = l; i <= r; ++i) {
		if (s.count(i)) ++ans;
		int x = 2 * i + 1;
		if (s.count(x)) ++ans;
	}
	
	cout << ans << '\n';
	
	return 0;
}