// 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 tbl[MAX];

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;
		}
	}
	tbl[2] = 1;
	for (i = 3; i <= ma; i += 2) {
		tbl[i] = tbl[i-1] + 1-notPrime[i];
		tbl[i+1] = tbl[i];
	}
}

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