
import java.util.Scanner;
public class Main {
public static void main(String[] args) {
Scanner input = new Scanner(System.in);
int N = input.nextInt();
int m = 0;
while(N >= 1)
{
if(N%10 > m)
{
m = N%10;
}
N = N / 10;
}
System.out.print(m);
input.close();
}
}


