#include<bits/stdc++.h>
using namespace std;
bool trust(int x){
	if(x<=1) return false;
	for(int i=2;i<=sqrt(x);i++){
		if(x%i==0){
			return false;
		}
	}
	return true;
}
inline int read() {
	int x=0,y=1;
	char c=getchar();
	while(c<'0'||c>'9') {
		if(c=='-')
			y=-1;
		c=getchar();
	}
	while(c<='9'&&c>='0') {
		x=x*10+c-'0',c=getchar();
	}
	return x*y;
}
void write(int x) {
	if(x<0) putchar('-');
	if(x>9) write(x / 10);
	putchar(x%10+'0');
}
int main(){
	long long ans=0;
	long long l=read();
	long long r=read();
	for(int i=l;i<=r;i++){
		if(trust(i)) ans++;
	}
	write(ans);
	return 0;
}