#include

#include

#include

#include

#include

using namespace std;

int primeTable[100001];

vector primeList;

void sieve(){

fill(primeTable,primeTable+100000,0);

primeTable[1]=1;

for(int i=2;i<=100000;i++){

if(primeTable[i]==0){

primeList.push_back(i);

for(int j=i*2;j<=100000;j+=i) primeTable[j]=1;

}

}

}

bool isPrime[1000001];

void fill_up(int n,int m){

//cout<

if(n==1) isPrime[0]=true;

for(int i=0;i

int start=n/primeList[i];

if(n%primeList[i]!=0) start++;

int end=m/primeList[i];

for(int j=max(2,start);j<=end;j++){

isPrime[j*primeList[i]-n]=true;

}

}

}

int main() {

/* Enter your code here. Read input from STDIN. Print output to STDOUT */

int n,m;

cin>>n>>m;

sieve();

fill_up(n,m);

int ans=0;

for(int i=0;i+2<=m-n;i++){

if(isPrime[i]==false && isPrime[i+2]==false) ans++;

}