import math
from collections import deque

def h(n):
    while n>9:
        n=reduce(lambda x,y:x+int(y),str(n),0)
    return n

def prime(max):
    max2 = (max-3)/2
    sieve = [True]*(max2+1)
    for i in range(int((math.sqrt(max)-3)/2)+1):
        if not sieve[i]: continue
        k = i+i+3
        j = k*(i+1)+i
        while j <= max2:
            sieve[j] = False
            j += k
    sieve = [2]+[i+i+3 for i in range(max2+1) if sieve[i]]
    return sieve

k=input()
n=input()
a=deque()
cnt=[0]*10
max=0
ans=0
for i,j in [(i,h(i)) for i in prime(n) if i>=k]:
    a.append((i,j))
    cnt[j]+=1
    while cnt[j]>1:cnt[a.popleft()[1]]-=1
    size=reduce(lambda x,y:x+y,cnt)
    if max<=size:
        max=size
        ans=a[0][0]
print ans