#include <bits/stdc++.h>
using namespace std;
#define rep(i, n) for(int i = 0; i < n; i++)
#define rep2(i, x, n) for(int i = x; i <= n; i++)
#define rep3(i, x, n) for(int i = x; i >= n; i--)
#define each(e, v) for(auto &e: v)
#define pb push_back
#define eb emplace_back
#define all(x) x.begin(), x.end()
#define rall(x) x.rbegin(), x.rend()
#define sz(x) (int)x.size()
using ll = long long;
using pii = pair<int, int>;
using pil = pair<int, ll>;
using pli = pair<ll, int>;
using pll = pair<ll, ll>;
const int MOD = 1000000007;
//const int MOD = 998244353;
const int inf = (1<<30)-1;
const ll INF = (1LL<<60)-1;
template<typename T> bool chmax(T &x, const T &y) {return (x < y)? (x = y, true) : false;};
template<typename T> bool chmin(T &x, const T &y) {return (x > y)? (x = y, true) : false;};

struct io_setup{
    io_setup(){
        ios_base::sync_with_stdio(false);
        cin.tie(NULL);
        cout << fixed << setprecision(15);
    }
} io_setup;

template<typename T>
vector<T> divisors(const T &n){
    vector<T> ret;
    for(T i = 1; i*i <= n; i++){
        if(n%i == 0){
            ret.push_back(i);
            if(i*i != n) ret.push_back(n/i);
        }
    }
    sort(begin(ret), end(ret));
    return ret;
}

template<typename T>
vector<pair<T, int>> prime_factor(T n){
    vector<pair<T, int>> ret;
    for(T i = 2; i*i <= n; i++){
        int cnt = 0;
        while(n%i == 0) cnt++, n /= i;
        if(cnt > 0) ret.emplace_back(i, cnt);
    }
    if(n > 1) ret.emplace_back(n, 1);
    return ret;
}

template<typename T>
bool is_prime(const T &n){
    if(n == 1) return false;
    for(T i = 2; i*i <= n; i++){
        if(n%i == 0) return false;
    }
    return true;
}

//1,2,...,nのうちkと互いに素である自然数の個数
template<typename T>
T coprime(T n, T k){
    vector<pair<T, int>> ps = prime_factor(k);
    int m = ps.size();
    T ret = 0;
    for(int i = 0; i < (1<<m); i++){
        T prd = 1;
        for(int j = 0; j < m; j++){
            if((i>>j)&1) prd *= ps[j].first;
        }
        ret += (__builtin_parity(i)? -1 : 1)*(n/prd);
    }
    return ret;
}

vector<bool> Eratosthenes(const int &n){
    vector<bool> ret(n+1, true);
    if(n >= 0) ret[0] = false;
    if(n >= 1) ret[1] = false;
    for(int i = 2; i*i <= n; i++){
        if(!ret[i]) continue;
        for(int j = i+i; j <= n; j += i) ret[j] = false;
    }
    return ret;
}

vector<int> Eratosthenes2(const int &n){
    vector<int> ret(n+1);
    iota(begin(ret), end(ret), 0);
    if(n >= 0) ret[0] = -1;
    if(n >= 1) ret[1] = -1;
    for(int i = 2; i*i <= n; i++){
        if(ret[i] < i) continue;
        for(int j = i+i; j <= n; j += i) ret[j] = min(ret[j], i);
    }
    return ret;
}

int main(){
    vector<bool> ps = Eratosthenes(2000000);

    int L, R; cin >> L >> R;

    int ans = 0;

    rep2(i, L, R){
        if(ps[i]) ans++;
        if(i+1 <= R && ps[2*i+1]) ans++;
    }

    cout << ans << '\n';
}