#include <bits/stdc++.h>
using namespace std;
#define ll long long
#define ld long double
#define REP(i,m,n) for(int i=(int)(m); i<(int)(n); i++)
#define rep(i,n) REP(i,0,n)
#define RREP(i,m,n) for(int i=(int)(m); i>=(int)(n); i--)
#define rrep(i,n) RREP(i,(n)-1,0)
#define all(v) v.begin(), v.end()
#define endk '\n'
const int inf = 1e9+7;
const ll longinf = 1LL<<60;
const ll mod = 1e9+7;
const ll mod2 = 998244353;
const ld eps = 1e-10;
template<typename T1, typename T2> inline void chmin(T1 &a, T2 b){if(a>b) a=b;}
template<typename T1, typename T2> inline void chmax(T1 &a, T2 b){if(a<b) a=b;}

bool arr[2000005];
vector<ll> primes;
void Eratosthenes(int N) {
  for(int i=0; i<=N; i++) {
    arr[i] = true;
  }
  for(int i=2; i<=N; i++) {
    if(arr[i]) {
      for(int j=0; i*(j+2)<=N; j++) {
        arr[i*(j+2)] = false;
      }
    }
  }
  for(int i=2; i<=N; i++) {
    if(arr[i]) {
      primes.push_back(i);
    }
  }
}
int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  int l, r; cin >> l >> r;
  Eratosthenes(2000005);
  int ans = 0;
  ans += upper_bound(all(primes), r) - lower_bound(all(primes), l);
  for(int a=l; a<r; a++) {
    int b = a+1;
    int sum = a+b;
    auto itr = lower_bound(all(primes), sum);
    if(itr == primes.end()) continue;
    if(*itr == sum) ans++;
  }
  cout << ans << endk;
  return 0;
}