def search_str(s:str) -> str: def seach_circle(left:int ,right:int) -> str: while(left>= 0 and right<len(s) and s[left]==s[right]): left -=1 right +=1 return s[left+1:right] long_str ='' for i in range(len(s)): temp = seach_circle(i,i) if(len(temp)>len(long_str)): long_str=temp temp = seach_circle(i,i+1) if(len(temp)>len(long_str)): long_str=temp return long_str target =input() print(search_str(target))