mod = 1000000007
eps = 10**-9


def main():
    import sys
    input = sys.stdin.readline

    NMAX = 3*10 ** 6
    is_prime = [1] * (NMAX + 1)
    is_prime[0] = is_prime[1] = 0
    for p in range(2, NMAX + 1):
        if not is_prime[p]:
            continue
        for d in range(2, NMAX + 1):
            if p * d > NMAX:
                break
            is_prime[p * d] = 0

    L, R = map(int, input().split())
    ans = 0
    for i in range(L, R+1):
        if is_prime[i]:
            ans += 1
        if is_prime[i * 2 + 1]:
            if i != R:
                ans += 1
    print(ans)


if __name__ == '__main__':
    main()