#include<bits/stdc++.h>
using namespace std;
//#include <atcoder/modint>
//using namespace atcoder;
//using mint = modint998244353;
//using mint = modint1000000007;
#pragma region template
using ll = long long;
template <class T> using V = vector<T>;
template <class T> using VV = V<V<T>>;
#define overload4(_1,_2,_3,_4,name,...) name
#define overload3(_1,_2,_3,name,...) name
#define rep1(n) for(ll i=0;i<n;++i)
#define rep2(i,n) for(ll i=0;i<n;++i)
#define rep3(i,a,b) for(ll i=a;i<b;++i)
#define rep(...) overload3(__VA_ARGS__,rep3,rep2,rep1)(__VA_ARGS__)
#define all(x) (x).begin(), (x).end()
#define rall(x) (x).rbegin(), (x).rend()
#define sz(x) ((int)(x).size())
#define pb push_back
#define eb emplace_back
#define fi first
#define se second
#define debug(var)  do{std::cerr << #var << " : ";view(var);}while(0)
template<typename T> void view(T e){std::cerr << e << std::endl;}
template<typename T> void view(const std::vector<T>& v){for(const auto& e : v){ std::cerr << e << " "; } std::cerr << std::endl;}
template<typename T> void view(const std::vector<std::vector<T> >& vv){cerr << endl;int cnt = 0;for(const auto& v : vv){cerr << cnt << "th : "; view(v); cnt++;} cerr << endl;}
std::ostream &operator<<(std::ostream &dest, __int128_t value) {
  std::ostream::sentry s(dest);
  if (s) {
    __uint128_t tmp = value < 0 ? -value : value;
    char buffer[128];
    char *d = std::end(buffer);
    do {
      --d;
      *d = "0123456789"[tmp % 10];
      tmp /= 10;
    } while (tmp != 0);
    if (value < 0) {
      --d;
      *d = '-';
    }
    int len = std::end(buffer) - d;
    if (dest.rdbuf()->sputn(d, len) != len) {
      dest.setstate(std::ios_base::badbit);
    }
  }
  return dest;
}
ll power(ll a, ll p){ll ret = 1; while(p){if(p & 1){ret = ret * a;} a = a * a; p >>= 1;} return ret;}
ll modpow(ll a, ll p, ll mod){ll ret = 1; while(p){if(p & 1){ret = ret * a % mod;} a = a * a % mod; p >>= 1;} return ret;}
template<class T, class K>bool chmax(T &a, const K b) { if (a<b) { a=b; return 1; } return 0; }
template<class T, class K>bool chmin(T &a, const K b) { if (b<a) { a=b; return 1; } return 0; }
#pragma endregion
int dx[]={1,0,-1,0};
int dy[]={0,1,0,-1};
const int inf = 1001001001;
const ll INF = 1001001001001001001ll;
// const double pi = acos(-1);
//const ll mod = 998244353;
const ll mod = 1000000007;

struct Sieve {
    int MX;
    vector<bool> ip; // is prime
    vector<int> mu; // mobius
    vector<int> mf; // min factor
    Sieve(int _MX, bool factor = false, bool mobius = false):MX(_MX){
        ip.resize(MX, true);
        if(factor) mf.resize(MX, 0);
        if(mobius) mu.resize(MX, 1);
        for(int i = 2; i < MX; i++){
            if(!ip[i]) continue;
            ip[i] = true;
            if(factor) mf[i] = i;
            if(mobius) mu[i] = -1;
            for(int j = i+i; j < MX; j += i){
                ip[j] = false;
                if(factor && mf[j] == 0) mf[j] = i;
                if(mobius){
                    if(j / i % i == 0) mu[j] = 0;
                    else mu[j] = -mu[j];
                }
            }
        }
    }

    vector<pair<int, int>> factorize(int n){
        vector<pair<int, int>> res;
        int p = mf[n], e = 0;
        while(n != 1){
            while(n % p == 0){
                n /= p;
                ++e;
            }
            res.emplace_back(p, e);
            p = mf[n], e = 0;
        }
        return res;
    }

    /* make 1-indexed vectors */
    // return F where F(i) = Σf(k) (i|k)
    template<class T> void fast_zeta(vector<T> &f){
        int N = f.size();
        for(int i = 2; i < N; i++){
            if(!ip[i]) continue;
            for(int j = (N - 1)/i; j >= 1; --j) f[j] += f[i*j];
        }
    }

    // return f where F(i) = Σf(k) (i|k)
    template<class T> void fast_mobius(vector<T> &F){
        int N = F.size();
        for(int i = 2; i < N; i++){
            if(!ip[i]) continue;
            for(int j = 1; j <= (N - 1)/i; ++j) F[j] -= F[i*j];
        }
    }

    // return h where h(k) = Σ f(i)*g(i) (gcd(i, j) = k) in O(NloglogN)
    template<class T> vector<T> gcd_convolution(const vector<T> &f, const vector<T> &g){
        int N = max(f.size(), g.size());
        vector<T> F(N), G(N), h(N);
        for(int i = 0; i < (int)f.size(); i++) F[i] = f[i];
        for(int i = 0; i < (int)g.size(); i++) G[i] = g[i];
        fast_zeta(F);
        fast_zeta(G);
        for(int i = 1; i < N; i++) h[i] = F[i] * G[i];
        fast_mobius(h);
        return h;
    }
};

int main(){
    cin.tie(nullptr);
    ios::sync_with_stdio(false);
    //cout << fixed << setprecision(20);
    ll l, r; cin >> l >> r;
    Sieve s(2000100);
    // (a + b)(b - a + 1) / 2
    int ans = 0;
    for(ll a = l; a <= r; a++){
        // b - a + 1 = 1 iff b = a
        if(a != 1 && s.ip[a]) ans++;
        // b - a + 1 = 2 iff b = a + 1
        if(a + 1 > r) continue;
        if(s.ip[2*a + 1]) ans++;
    }
    cout << ans << endl;
}