include <stdio.h>

int main() {

int n, m, f = 0, sum = 0;
scanf("%d%d", &n, &m);
for (int i = n; i <= m; i++) {
	f = 0;
	for (int j = 2; j <= i - 1; j++) {
		if (i % j == 0) {
			f = 1;
			break;
		}
	}
	if (f == 0) {
		sum++;
	}
}
printf("%d", sum);

return 0;

}