l,r=map(int,input().split())
ans=0
import math
def isprime(n):
    if n < 2:
        return False
    if n == 2:
        return True
    if n % 2 == 0:
        return False
    for i in range(3, int(math.sqrt(n)) + 1, 2):
        if n % i == 0: return False
    return True
for a in range(l,r+1):
    if isprime(a):
        ans+=1
    if isprime(2*a+1):
        ans+=1
print(ans)