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))