#include <bits/stdc++.h>
using namespace std;
#include <atcoder/all>
using namespace atcoder;

#define ll long long
#define rep(i, n) for (int i = 0; i < (n); i++)
#define P pair<int, int>
#define LP pair<ll, ll>
#define fi first
#define se second
#define pb push_back
#define eb emplace_back
#define all(s) s.begin(), s.end()
#define rall(s) s.rbegin(), s.rend()
template<class T>
void chmax(T& a, T b) { a = max(a, b); };
template<class T>
void chmin(T& a, T b) { a = min(a, b); };

int main() {
  int l, r;
  cin >> l >> r;
  vector<bool> prime(2*r+1, true);
  prime[1] = false;
  for (int i = 2; i <= 2*r; i++) {
    if (prime[i]) {
      for (int j = i*2; j <= 2*r; j += i) prime[j] = false;
    }
  }
  int ans = 0;
  for (int i = l; i <= r; i++) {
    if (prime[i]) ans++;
  }
  for (int i = l; i < r; i++) {
    if (prime[2*i+1]) ans++;
  }
  cout << ans << endl;




  return 0;
}