#include <bits/stdc++.h>
#include <atcoder/all>
using namespace std;
using namespace atcoder;
typedef int64_t lint;
#define rep(i, n) for(int i=0; i<n; i++)
#define repx(i, l, n) for(int i=l; i<n; i++)
#define all(v) v.begin(), v.end()
#define show(x) cout << #x << ": " << x << endl;
#define list(x) cout << #x << ": " << x << "  ";
#define pb push_back
using vi = vector<lint>;
using vvi = vector<vector<lint>>;
template<class T> inline void vin(vector<T>& v) { rep(i, v.size()) cin >> v.at(i); }
template<class T> inline bool chmax(T& a, T b) { if (a < b) { a = b; return 1; } return 0; }
template<class T> inline bool chmin(T& a, T b) { if (a > b) { a = b; return 1; } return 0; }
template<class T> inline void drop(T x) { cout << x << endl; exit(0); }
template<class T> void vout(vector<T> v) { rep(i, v.size()) { cout << v.at(i) << ' '; } cout << endl; }
constexpr lint LINF = LLONG_MAX/2;
struct Sieve {
	int n;
	vector<lint> p;
	Sieve(int n=1):n(n), p(n+1) {
		p.at(0) = -1, p.at(1) = -1;
		for(lint i=2; i<=sqrt(n)+1; i++) {
			if(!p.at(i)) {
				p.at(i) = i;
				for(lint j=i*i; j<=n; j+=i) {
					if (!p.at(j)) p.at(j) = i;
				}
			}
		}
	}
	bool isPrime(int x) { return (p.at(x) == x || p.at(x) == 0);}
	vector<lint> factorList(int x) {
		vector<lint> v;
		while (x != 1) {
			v.push_back(p.at(x));
			x /= p.at(x);
		}
		return v;
	}
	vector<lint> factor(int x) {
		vector<lint> v;
		while (x != 1) {
			int t = p.at(x);
			v.push_back(t);
			while (x % t == 0) x /= t;
		}
		return v;
	}
};

int main() {
	lint L, R;
	cin >> L >> R;

	lint a=0, b=0, c=0, x, y, z;
	Sieve S(R*2+1);
	repx(i, L, R+1) {
		if (S.isPrime(i)) a++;
		if (S.isPrime(i*2+1) && i != R) a++;
	}
	std::cout << a << '\n';
}