#include <algorithm>
#include <bitset>
#include <cassert>
#include <cmath>
#include <cstdio>
#include <fstream>
#include <functional>
#include <iomanip>
#include <iostream>
#include <iterator>
#include <map>
#include <numeric>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <unordered_set>
using namespace std;
#if __has_include(<atcoder/all>)
#include <atcoder/all>
using namespace atcoder;
#endif
#define GET_MACRO(_1, _2, _3, NAME, ...) NAME
#define _rep(i, n) _rep2(i, 0, n)
#define _rep2(i, a, b) for(int i = (int)(a); i < (int)(b); i++)
#define rep(...) GET_MACRO(__VA_ARGS__, _rep2, _rep)(__VA_ARGS__)
#define all(x) (x).begin(), (x).end()
#define rall(x) (x).rbegin(), (x).rend()
using i64 = long long;
template<class T, class U>
bool chmin(T& a, const U& b) { return (b < a) ? (a = b, true) : false; }
template<class T, class U>
bool chmax(T& a, const U& b) { return (b > a) ? (a = b, true) : false; }

template<typename T>istream& operator>>(istream&i,vector<T>&v){rep(j,v.size())i>>v[j];return i;}
template<typename T>string join(vector<T>&v){stringstream s;rep(i,v.size())s<<' '<<v[i];return s.str().substr(1);}
template<typename T>ostream& operator<<(ostream&o,vector<T>&v){if(v.size())o<<join(v);return o;}
template<typename T>string join(vector<vector<T>>&vv){string s="\n";rep(i,vv.size())s+=join(vv[i])+"\n";return s;}
template<typename T>ostream& operator<<(ostream&o,vector<vector<T>>&vv){if(vv.size())o<<join(vv);return o;}

vector<int> osa_k(int n)
{
    vector<int> res(n + 2);
    iota(res.begin(), res.end(), 0);
    for (int i = 4; i <= n; i += 2) res[i] = 2;
    for (int i = 9; i <= n; i += 6) res[i] = 3;
    for (int i = 6; i <= n; i += 6)
    {
        if (res[i - 1] == i - 1)
        {
            int w = (i - 1);
            if (n / w >= w)
            {
                for (int j = w * w; j <= n; j += 2 * w)
                {
                    if (res[j] == j) res[j] = w;
                }
            }
        }
        if (res[i + 1] == i + 1)
        {
            int w = (i + 1);
            if (n / w >= w)
            {
                for (int j = w * w; j <= n; j += 2 * w)
                {
                    if (res[j] == j) res[j] = w;
                }
            }
        }
    }
    return res;
}

auto table = osa_k(2000100);

int main(){
    int l, r;
    cin >> l >> r;
    table[1] = 0;
    set<int> prime;
    rep(i, 2000100){
        if (table[i] == i) prime.insert(i);
    }
    i64 ans = 0;
    for (int a = l; a <= r; a++){
        if (prime.count(a)) ans++;
        if (a + 1 <= r){
            if (prime.count(2 * a + 1)) ans++;
        }
    }
    cout << ans << endl;
}