def is_prime(x):
    if x == 2:
        return True
    if x == 1 or x % 2 == 0:
        return False
    for k in range(3, int(x ** 0.5) + 1, 2):
        if x % k == 0:
            return False
    return True


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


ans = 0
for i in range(1, 10 ** 6 + 1):
    a = i
    b = i + 1
    if l <= a <= r and l <= b <= r:
        ab = a + b
        if is_prime(ab):
            ans += 1
    a = i
    b = i
    if l <= a <= r and l <= b <= r:
        if is_prime(a):
            ans += 1
print(ans)