from math import sqrt
k = input()
n = input()

prime = [1]*(n+1)
prime[0] = prime[1] = 0
for i in xrange(2, int(sqrt(n))+1):
    if prime[i]:
        for j in xrange(i*i, n+1, i):
            prime[j] = 0

def h(x):
    while x>9:
        x = sum(map(int, `x`))
    return x

used = [0]*10
ps = [x for x in xrange(k, n+1) if prime[x]]
w = -1
ma = 0
ans = -1
for i, x in enumerate(ps):
    y = h(x)
    if used[y]:
        while used[y]:
            w += 1
            used[h(ps[w])] = 0
    used[y] = 1
    cnt = i - w
    if ma <= cnt:
        ma = cnt
        ans = ps[w+1]
print ans