p,q = input().split()
def kiem_tra(start,stop):
count = 0
for num in range (start,stop+1):
if num == 2:
count+=1
elif num < 2:
break
else:
for i in range (2,int(num**(1/2)+1)):
if num%i == 0:
break
else:
count+=1
return count
print(kiem_tra(int(p),int(q)))