// yuki 1657 Sum is Prime (Easy Version)
// 2021.8.28

#include <stdio.h>
#include <math.h>

#define MAX  2000005
char notPrime[MAX] = { 1,1,0,0,1 };	// zero: if prime
int prime[150000], sz;

void sieve(int ma) {
	int i, j, sq = sqrt(ma);
	for (i = 3; i <= sq; i += 2) {
		if (!notPrime[i]) {
			for (j = i * i; j <= ma; j += i) notPrime[j] = 1;
		}
	}
	sz = 1, prime[0] = 2;
	for (i = 3; i <= ma; i += 2) if (!notPrime[i])
		prime[sz++] = i;
}

// 見つからなければ、一つ小さい要素を返す
int bs(int x) {
	int l = 0, r = sz;
    while (l < r) {
        int m = (l+r) >> 1;
        if (prime[m] <= x) l = m + 1; else r = m;
    }
	return l-1;
}

int main() {
	int L, R, ans;
	scanf("%d%d", &L, &R);
	sieve(2*R);
	ans = bs(R)-bs(L-1)+bs(2*R-1)-bs(2*L);
	printf("%d\n", ans);
	return 0;
}