#include <iostream>
#include <cstdio>
#include <cstring>
using namespace std;
int main() {
int a[30]={0}, flag=0;
char s[15];
scanf("%s", s);
for(int i=0; i<strlen(s); i++) a[s[i]-'a']++;
for(int i=27; i>=0; i--) if(a[i]>0 && flag==0) while(a[i]>0) printf("%c", i+'a'), a[i]--, flag=1;
return 0;
}
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