#55170: python
fredacer3066@gmail.com
(Cool Chen)
import math
while True:
try:
number=int(0)
a,b=map(int,input().split())
for i in range(a,b+1):
if i!=1:#排除1
for p in range(2,int(math.sqrt(i)+1)):
if i%p==0:
break#不是質數就退出
else :
number+=1#質數+1
print(number)
except EOFError:
break