import java.util.*;
public class A {
public static void main(String args[]) {
Scanner sc = new Scanner(System.in);
//prime Sieve Algo
int[] arr = new int[100000];
for(int i=3;i<=100000;i+=2){// odd number 1
arr[i] = 1;
}
for(int i=3;i<100000;i+=2){
if(arr[i] == 1){
System.out.println(i);
for (int j=i*i;j < 100000 ; j = j+i) {
arr[j] = 0 ;
}
}
}
arr[0] = 0;
arr[1] = 1;
arr[2] = 1;
// for (int temp :arr ) {
// System.out.print(temp+" ");
// }
int start =sc.nextInt();
int end = sc.nextInt();
int count =0;
for(int i=start;i<end;i++){
if(arr[i] == 1){
count++;
}
}
System.out.println(count+" ");
}
}