def hash(a):
    a =str(a)
    s =0
    for i in a:
        s += int(i)
    if s >= 10:
        return hash(s)
    else:
        return s

def hantei(a):
    s =True
    for i in range(3,int(a**0.5),2):
        if a%i == 0:
            s =False
            break
    if a!=2 and a%2 ==0:
        s =False
    return s

K,N =[int(input()) for _ in range(2)]
l=[]
n=0
for i in range(K,N+1):
    if hantei(i):
        m =hash(i)
        if not m in l:
            l.append(m)
        else:
            l =l[l.index(m)+1:]
            l.append(m)
            if n < len(l):
                n = len(l)
print(n)