IsPrime = [True for _ in range(2 * 10**5 + 1)]
IsPrime[0] = IsPrime[1] = False
for i in range(2, 2 * 10**5 + 1):
    if IsPrime[i]:
        for j in range(i + i, 2 * 10**5 + 1, i):
            IsPrime[j] = False

n = int(input())
k = int(input())

from collections import defaultdict

Prime = []
L = []
for i in range(n, k + 1):
    if IsPrime[i]:
        Prime.append(i)
        ic = i
        while ic > 9:
            temp = 0
            while ic != 0:
                temp += ic % 10
                ic //= 10
            ic = temp
        L.append(ic)

m = len(Prime)
counter = defaultdict(int)
DP = [0 for _ in range(m)]
right = 0
length = 0
ans = 0
for left in range(m):
    while right < m:
        counter[L[right]] += 1
        if counter[L[right]] == 2:
            counter[L[right]] -= 1
            break
        right += 1
    if right - left >= length:
        length = right - left
        ans = max(ans, Prime[left])
    counter[L[left]] -= 1
print(ans)