#include <bits/stdc++.h>

#define rep(i,n) for(int i=0;i<(int)(n);i++)
#define FOR(i,n,m) for(int i=(int)(n); i<=(int)(m); i++)
#define RFOR(i,n,m) for(int i=(int)(n); i>=(int)(m); i--)
#define ITR(x,c) for(__typeof(c.begin()) x=c.begin();x!=c.end();x++)
#define RITR(x,c) for(__typeof(c.rbegin()) x=c.rbegin();x!=c.rend();x++)
#define setp(n) fixed << setprecision(n)

template<class T> bool chmax(T &a, const T &b) { if (a<b) { a=b; return 1; } return 0; }
template<class T> bool chmin(T &a, const T &b) { if (a>b) { a=b; return 1; } return 0; }

#define ll long long
#define vll vector<ll>
#define vi vector<int>
#define pll pair<ll,ll>
#define pi pair<int,int>

#define all(a) (a.begin()),(a.end())
#define rall(a) (a.rbegin()),(a.rend())
#define fi first
#define se second
#define pb push_back
#define ins insert

#define debug(a) cerr<<(a)<<endl
#define dbrep(a,n) rep(_i,n) cerr<<(a[_i])<<" "; cerr<<endl
#define dbrep2(a,n,m) rep(_i,n){rep(_j,m) cerr<<(a[_i][_j])<<" "; cerr<<endl;}

using namespace std;

template<class A, class B>
ostream &operator<<(ostream &os, const pair<A,B> &p){return os<<"("<<p.fi<<","<<p.se<<")";}
template<class A, class B>
istream &operator>>(istream &is, pair<A,B> &p){return is>>p.fi>>p.se;}

template<class T>
vector<T> make_vec(size_t a){
	return vector<T>(a);
}
template<class T, class... Ts>
auto make_vec(size_t a, Ts... ts){
	return vector<decltype(make_vec<T>(ts...))>(a, make_vec<T>(ts...));
}

//-------------------------------------------------
//--Sieve of Eratosthenes
//-------------------------------------------------
class Sieve
{
private:
    vector<int> minf;
public:
    Sieve(int n):minf(n+1){
        iota(minf.begin(),minf.end(),0);
        for(int i=2; i*i<=n; i++){
            if (minf[i]!=i) continue;
            for(int j=i*2; j<=n; j+=i){
                if (minf[j]==j) minf[j] = i;
            }
        }
    }
    using P = pair<int,int>;
    vector<P> factor(int N){
        vector<P> ret;
        while(N>1){
            int f = minf[N];
            int cnt = 0;
            while(minf[N]==f){
                N/=f;
                cnt++;
            }
            ret.emplace_back(f,cnt);
        }
        return ret;
    }
    bool operator[](int x){return minf[x]==x;}
};

//-------------------------------------------------

int main(void)
{
    cin.tie(0);
    ios::sync_with_stdio(false);
	int L,R; cin>>L>>R;

	Sieve sieve(R*2);
	if (L==R){
		if (sieve[L]) cout<<"1\n";
		else cout<<"0\n";
		return 0;
	}
	
	int ans=0;
	FOR(i,L,R){
		if (sieve[i] && i>1) ans++;
		if (sieve[2*i-1] && 2*i-1>1) ans++;
	}
	cout<<ans<<"\n";
    return 0;
}