#include <iostream>
#include <vector>
#include <algorithm>
#include <functional>
#include <queue>
#include <map>
#include <cmath>
#include <iomanip>

using namespace std;

int main() {

  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  //cout << fixed << setprecision(15);
  
  int l , r;
  cin >> l >> r;
  vector<int> prime(2200000 , 0);
  prime[1] = 2;
  for (int i = 2; i <= 2000000; i++) {
    for (int j = i; j <= 2000000; j += i) {
      prime[j]++;
    }
  }
  long long ans = 0;
  for (int i = l; i <= r; i++) {
    if (prime[i] <= 1) ans++;
  }
  for (int i = l; i <= r - 1; i++) {
    if (prime[i + i + 1] <= 1) ans++;
  }
  cout << ans << endl;
  return 0;
}