#include <bits/stdc++.h>
#define rep(i,n) for(int i=0;i<(int)(n);i++)
#define chmin(x,y) x = min((x),(y))
#define chmax(x,y) x = max((x),(y))
#define popcount(x) __builtin_popcountll(x)
using namespace std;
using ll = long long ;
using P = pair<int,int> ;
using pll = pair<long long,long long>;
const int INF = 1e9;
const long long LINF = 1e17;
const int MOD = 1000000007;
//const int MOD = 998244353;
const double PI = 3.14159265358979323846;

int main(){
    int mx = 2000005;
    int L,R;
    cin >> L >> R;

    vector<int> isprime(mx+1,-1);
    isprime[0] = isprime[1] = -1;
    for(int i=2;i<=mx;i++){
        if (isprime[i] == -1) {
            isprime[i] = i;
            for(int j=2*i;j<=mx;j+=i){
                isprime[j] = i;
            }
        }
    }

    int ans = 0;
    for(int i=L;i<=R;i++){
        if (isprime[i] == i) ++ans;
        if (i < R && isprime[2*i+1] == 2*i+1) ++ans;
    }
    cout << ans << endl;
    return 0;
}