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

def seachPrimeNum(N):
    max = int(N**0.5)
    seachList = [i for i in range(2,N+1)]
    primeNum = []
    while seachList[0] <= max:
        primeNum.append(seachList[0])
        tmp = seachList[0]
        seachList = [i for i in seachList if i % tmp != 0]
    primeNum.extend(seachList)
    return primeNum

P = set(seachPrimeNum(2*R))
ans = 0
for i in range(L,R):
    if i in P:
        ans += 1
    if 2*i+1 in P:
        ans += 1
if R in P:
    ans += 1
print(ans)