def is_prime(a):
    if a < 2:
        return False
    if a in (2, 3):
        return True
    if a % 2 == 0 or a % 3 == 0:
        return False

    # Miller-Rabin test with specific bases for 32-bit integers
    d, s = a - 1, 0
    while d % 2 == 0:
        d //= 2
        s += 1

    for i in [2, 7, 61]:
        if i >= a:
            continue
        x = pow(i, d, a)
        if x == 1 or x == a - 1:
            continue
        for j in range(s):
            x = pow(x, 2, a)
            if x == a - 1:
                break
        else:
            return False
    return True


while True:
    try:
        a, b = map(int, input().split())
        c = 0
        for i in range(a, b + 1):
            if is_prime(i):
                c += 1
        print(c)
    except EOFError:
        break

def is_prime(a):
    if a < 2:
        return False
    if a in (2, 3):
        return True
    if a % 2 == 0 or a % 3 == 0:
        return False

    # Miller-Rabin test with specific bases for 32-bit integers
    d, s = a - 1, 0
    while d % 2 == 0:
        d //= 2
        s += 1

    for i in [2, 7, 61]:
        if i >= a:
            continue
        x = pow(i, d, a)
        if x == 1 or x == a - 1:
            continue
        for j in range(s):
            x = pow(x, 2, a)
            if x == a - 1:
                break
        else:
            return False
    return True


while True:
    try:
        a, b = map(int, input().split())
        c = 0
        for i in range(a, b + 1):
            if is_prime(i):
                c += 1
        print(c)
    except EOFError:
        break

miller rabin是一種無恥快速的解法