#include<bits/stdc++.h>
using namespace std;
bool Prime[2143483647];
inline int qread(){
	register int x = 0,f = 1;
	register char ch = getchar();
	while(ch<'0'||ch>'9'){
		if(ch == '-'){
			f = -1;
		}
		ch = getchar();
	}
	while(ch>='0'&&ch<='9'){
		x = x * 10 + ch - '0';
		ch = getchar();
	}
	return x*f;
}
inline void qwrite(int x){
	if(x<0){
		putchar('-');
		x = -x;
	}
	int n=0;
	char ch[100];
	while(x||!n){
		ch[++n] = x % 10 + '0';
		x /= 10;
	}
	for(int i=n;i>=1;i--){
		putchar(ch[i]);
	}
	return;
}
void isPrime(int n){
	for(int i=1;i<=n;i++){
		Prime[i] = true;
	}
	Prime[1] = false;
	for(register int i=2;i*i<=n;i++){
        if(Prime[i]){
        	for(register int j=i*i;j<=n;j+=i){
        		Prime[j] = false;
			}
		}
    }
}
int main(){
	int l = qread(),r = qread(),ans = 0;
	isPrime(r);
	for(int i=l;i<=r;i++){
		if(Prime[i]){
			ans++;
		}
	}
	qwrite(ans);
	return 0;
}

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