l, r = map(int, input().split())

max_ = 2 * r
isprime = [True] * max_
isprime[0] = isprime[1] = False
for i in range(2, int(max_ ** 0.5 + 1)):
    if not isprime[i]: continue
    for j in range(i * i, max_, i):
        isprime[j] = False
        
ans = 0
for i in range(l, r):
    if isprime[i]:
        ans += 1
    if isprime[2 * i + 1]:
        ans += 1
if isprime[r]:
    ans += 1
print(ans)