import math

def prime(x):
    if x <= 1:
        return False
    for i in range(2, int(math.sqrt(x)) + 1):
        if x % i == 0:
            return False
    return True

def run():
    a, b = map(int, input().split())
    c = 0
    
    for n in range(a, b + 1):
        if prime(n - 1) and prime(n + 1):
            c += 1
    
    print(c)
 run()
Prime// Flipkart