#include <bits/stdc++.h>
using namespace std;
#define rep(i, n) for (int i = 0; i < (n); ++i)
using ll = long long;
using ull = unsigned long long;

int main() {
  cin.tie(nullptr)->sync_with_stdio(false);
  constexpr int n = 3e6;
  vector<bool> is_prime(n + 1, true);
  is_prime[0] = is_prime[1] = false;
  for (int i = 2; i * i <= n; ++i) {
    if (is_prime[i]) {
      for (int j = i * i; j <= n; j += i) is_prime[j] = false;
    }
  }
  int l, r;
  cin >> l >> r;
  int ans = 0;
  for (int i = l; i <= r; ++i) ans += is_prime[i];
  for (int i = l; i < r; ++i) ans += is_prime[2 * i + 1];
  cout << ans << '\n';
  return 0;
}