#include <iostream>
#include <string>

using namespace std;

string s;
char minb[200000], maxb[200000];
int nmin[200000], nmax[200000], mind, maxk, n, idx;

int main()
{
    cin >> s;
    n = s.size();
    minb[0] = s[0];
    nmin[0] = 0;
    maxb[0] = s[0];
    nmax[0] = 0;
    for (int i = 1; i<n; i++)
    {
        if (s[i] <= minb[i - 1])
        {
            minb[i] = s[i];
            nmin[i] = i;
        }
        else
        {
            minb[i] = minb[i - 1];
            nmin[i] = nmin[i - 1];
        }
        if (s[i] >= maxb[i - 1])
        {
            maxb[i] = s[i];
            nmax[i] = i;
        }
        else
        {
            maxb[i] = maxb[i - 1];
            nmax[i] = nmax[i - 1];
        }
    }

    maxk = 0;
    mind = 1000000;
    idx = n - 1;

    for (int i = n - 2; i >= 0; i--)
    {
        if (abs(int(s[i] - minb[i])) == maxk && mind>abs(i - nmin[i] + 1) || abs(int(s[i] - minb[i]))>maxk)
        {
            idx = nmin[i];
            mind = abs(i - nmin[i]) + 1;
            maxk = abs(int(s[i] - minb[i]));
        }
        if (abs(int(s[i] - maxb[i])) == maxk && mind>abs(i - nmax[i] + 1) || abs(int(s[i] - maxb[i]))>maxk)
        {
            idx = nmax[i];
            mind = abs(i - nmax[i]) + 1;
            maxk = abs(int(s[i] - maxb[i]));
        }
    }
    cout << s.substr(idx, mind);
}