#include <bits/stdc++.h>
using namespace std;
const int MAX = 2000002;
int main(){
  vector<bool> prime(MAX, true);
  prime[1] = false;
  for (int i = 2; i < MAX; i++){
    if (prime[i]){
      for (int j = i * 2; j < MAX; j += i){
        prime[j] = false;
      }
    }
  }
  int L, R;
  cin >> L >> R;
  int ans = 0;
  for (int i = L; i <= R; i++){
    if (prime[i]){
      ans++;
    }
  }
  for (int i = L; i < R; i++){
    if (prime[i + (i + 1)]){
      ans++;
    }
  }
  cout << ans << endl;
}