Prime = [True for _ in range(2 * 10**6 + 2)]
Prime[0] = Prime[1] = False
for i in range(2, 2 * 10**6 + 2):
    if not Prime[i]:
        continue
    for j in range(2 * i, 2 * 10**6 + 2, i):
        Prime[j] = False

l, r = map(int, input().split())
ans = 0
for i in range(l, r + 1):
    if Prime[i]:
        ans += 1
    if i < r and Prime[2 * i + 1]:
        ans += 1
print(ans)