#include <bits/stdc++.h>
using namespace std;
#define rep(i, x, limit) for (long long i = (long long)x; i < (long long)limit; i++)
#define REP(i, x, limit) for (long long i = (long long)x; i <= (long long)limit; i++)
#define all(x) x.begin(), x.end()
#define rall(x) x.rbegin(), x.rend()
#define el endl
#define spa " "
#define Yes cout << "Yes" << el
#define No cout << "No" << el
#define YES cout << "YES" << el
#define NO cout << "NO" << el
#define inp(x) for(auto &i:x)cin>>i
#define eps (1e-10)
#define Equals(a,b) (fabs((a) - (b)) < eps )
#define debug(x) cerr << #x << " = " << x << el
using ll = long long;
using ull = unsigned long long;
using i128 = __int128_t;
using pii = pair<int,int>;
using pll = pair<ll,ll>;
using vi = vector<int>;
using vl = vector<ll>;
using vvl = vector<vector<ll>>;
using vs = vector<string>;
using vb = vector<bool>;
const double pi = 3.141592653589793238;
const int inf = 1073741823;
const ll infl = 1LL << 60;
const string ABC = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
const string abc = "abcdefghijklmnopqrstuvwxyz";
const ll MOD = 998244353;
#include<atcoder/all>
using namespace atcoder;
using mint = modint998244353;
using vm = vector<mint>;
#include <bits/stdc++.h>
using namespace std;

// 10^7 までの素数を列挙して返す（高速・省メモリ：奇数のみビット圧縮）
vector<int> sieve_primes(int n) {
    vector<int> primes;
    if (n < 2) return primes;
    primes.push_back(2);

    const size_t max_idx = (n - 1) / 2;
    vector<uint64_t> bits((max_idx + 64) >> 6, 0);

    auto get = [&](size_t i) -> bool {
        return (bits[i >> 6] >> (i & 63)) & 1ull;
    };
    auto set1 = [&](size_t i) {
        bits[i >> 6] |= (1ull << (i & 63));
    };

    for (size_t i = 1; (2ull*i + 1) * (2ull*i + 1) <= (unsigned)n; ++i) {
        if (get(i)) continue;
        uint64_t p = 2ull*i + 1;
        uint64_t start = (p*p - 1) / 2;
        for (uint64_t j = start; j <= max_idx; j += p) set1((size_t)j);
    }

    primes.reserve((size_t)(n / log(max(3, n)) * 1.1));
    for (size_t i = 1; i <= max_idx; ++i) {
        if (!get(i)) primes.push_back((int)(2*i + 1));
    }
    return primes;
}

/* 使用例
int main() {
    int N = 10000000;
    auto ps = sieve_primes(N);
    cout << ps.size() << "\n";
    // cout << ps.back() << "\n";
}
*/

int main() {
  ios::sync_with_stdio(false);
  cin.tie(nullptr);
  //(A+B)*(B-A+1)/2
  //2A+1
  //L<=A<=R-1,2L+1<=2A+1<=2R-1
  ll l,r;cin>>l>>r;
  auto P=sieve_primes(2*r);
  auto C=[&](ll x,ll y){
    return upper_bound(all(P),y)-upper_bound(all(P),x-1);
  };
  cout<<C(2*l+1,2*r-1)+C(l,r)<<el;
}