/*
 * count the primes.
 */
#include <stdio.h>
#include <stdlib.h>

#define P 4194400

char primes [P];

int
main (void)
{
	int i, j;
	int a, b, count;
	char s [100];

	for (i = 0; i < P; i++)
		primes [i] = 1;


	primes [0] = 0;
	primes [1] = 0;
	
	for (i = 2; i < P; i++) {
		for (j = i + i; j < P; j+= i)
			primes [j] = 0;
	}

	while (gets (s) != NULL) {
		sscanf (s, "%d %d", &a, &b);

		count = 0;

		for (i = a; i <= b; i++) {
			count += primes [i];
		}

		printf ("%d\n", count);
	}

	return 0;
}