L, R = map(int, input().split())

if R == 1:
    print(0)
    exit()

V = 3*R

prime = [True for i in range(V+1)]
p = 2
while (p * p <= V):
    if (prime[p] == True):
        for i in range(p * p, V+1, p):
            prime[i] = False
    p += 1
dee = []
primes = set()
for p in range(2, V+1):
    if prime[p]:
        dee.append(p)
        primes.add(p)

count = 0

for i in range (1, R+1):
    if L <= i <= R and i in primes:
        count+=1
    if L <= i and i+1 <= R and (2*i+1) in primes:
        count+=1
print(count)