string longestPalindrome(string s) {

if(s.empty() || s.length() < 1) return "";

int start = 0;

int end = 0;

for(int i=0;i<s.length();i++){

int len1 = MiddleExpand(s,i,i);

int len2 = MiddleExpand(s,i,i+1);

int len = max(len1,len2);

if(len > end - start){

start = i - ((len-1)/2);

end = i + (len/2);

}

}

return s.substr(start,end-start+1);

}

int MiddleExpand(string s,int left,int right){

if(s.empty() || left > right) return 0;

while(left >= 0 && right < s.length() && s.at(left)==s.at(right)){

left--;

right++;

}

return right-left-1;

}

string s;

getline(cin,s);

Solution sol;

cout<<sol.longestPalindrome(s);

}