import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.math.BigInteger;
class Main {
public static void main(String[] args) throws IOException {
BufferedReader br=new BufferedReader(new InputStreamReader(System.in));
String s;
while (!(s=br.readLine()).equals("0")) {
BigInteger largestPrime=BigInteger.ZERO;
for (int start=0;start<s.length();start++) {
for (int end=start+1;end<=s.length() && end<=start+5;end++) {
BigInteger test=new BigInteger(s.substring(start,end));
if (test.isProbablePrime(10) && test.compareTo(largestPrime)>0) {
largestPrime=test;
}
}
}
System.out.println(largestPrime);
}
}
}
