#include<bits/stdc++.h>
#define ll long long
using namespace std;
string a,b;
int main()
{
    cin>>a;
    int len=a.size();
    for(int i=1;i<=len/3;++i)
    {
        string s1=a.substr(0,i);
        string s2=a.substr(len-i,i);
        string s3=a.substr(i,len-2*i);
        if(s1==s2&&s3.find(s1)!=s3.npos)
            b=s1;
    }
    cout<<b<<endl;
    return 0;
}