from itertools import combinations
f = False
li = []
count = 0
def is_prime(n):
    if n > 1:
        for i in range(2, n):
            if (n % i) == 0:
                f = True
                break
        else:
            li.append(n)
    return li
a, b = map(int,input().split())
for i in range(a, b+1):
    k = is_prime(i)
com = combinations(k,2)
for c in com:
    if c[1] - c[0] == 6:
        count +=1
print(count)