#include <stdio.h>

int main()
{
	int L, R;
	scanf("%d %d", &L, &R);
	
	char flag[2000001] = {1, 1};
	int i, j, ans = 0;
	for (i = 2; i <= 2000000; i++) {
		if (flag[i] != 0) continue;
		for (j = i * 2; j <= 2000000; j += i) flag[j] = 1;
	}
	for (i = L; i <= R; i++) {
		if (flag[i] == 0) ans++;
		if (i < R && flag[i*2+1] == 0) ans++;
	}
	printf("%d\n", ans);
	fflush(stdout);
	return 0;
}