代码：

#include <bits/stdc++.h>
#define int long long 
using namespace std;
const int MAXN =100000010;
int primes[MAXN], ans, n;
bool st[MAXN];
inline int read(){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<<1)+(x<<3)+(ch^48);ch=getchar();}return x*f;}
void write(int x){if(x<0)putchar('-'),x=-x;if(x>9)write(x/10);putchar(x%10+'0');}
void eular(int MAXL) {
    for(register int i=2; i<=MAXL; i++) {
        if(!st[i]) primes[++ans]=i;
        for(register int j=1; primes[j]*i<=n; j++) {
            st[primes[j]*i]=true;
            if(i%primes[j]==0) break;
        }
    }
}
signed main() {
	int a,b;
	a=read(),b=read();
	n = b;
	eular(b);
    int cnt = 0;
    for(register int i=1; i<=ans; i++){
        if(primes[i] >= a && primes[i] <= b){
            ++cnt;
        }
    }
	write(cnt);
    return 0;
}