from bisect import *

def era(n):
    is_prime = [True] * (n+1)
    is_prime[0] = False
    is_prime[1] = False
    
    for i in range(2, int(n**0.5)+1):
        if not is_prime[i]:
            continue
        for j in range(2*i, n+1, i):
            is_prime[j] = False

    return is_prime, [i for i in range(n+1) if is_prime[i]]

N, P = map(int, input().split())
is_prime, primes = era(N)

if P == 1 or (P >= N//2+1 and is_prime[P]):
    print(1)
else:
    print(N - 1 - (len(primes)-bisect_left(primes, N//2+1)))