import math
a=int(input())
b=int(input())
def p(x):
	if x==2:
		return 0
	for i in range(3,math.sqrt(x)):
		if x%i==0:
			return 0
	return 1
cnt=0
for i in range(a+1,b):
	if p(i):
		cnt+=1
print(cnt)