#include<stdio.h> 
int suShu(int x){
	int i,c=0;
	for(i = 2;i <= x / 2;i++){
		if(x % i == 0){
			c++;
			break;
		}
	}
	
	return c;
	
}

int suShuLiang(int m,int n){
	int i,c=0;
	for(i = m;i <= n;i++){
		if(i > 1){
			if(suShu(i) == 0){
				c++;
			}
		}
	}
	return c;	
}

int main(){ 
	int m,n;
	scanf("%d %d",&m,&n);
	printf("%d",suShuLiang(m,n));
	return 0;
} 

/**************************************************************
	Problem: 1063
	User: admin
	Language: C
	Result: Accepted
	Time:72 ms
	Memory:1144 kb
****************************************************************/