#include <bits/stdc++.h>
std::string s ;
std::map <char, int> mp ;
int main(){
    freopen("ordine.in", "r", stdin) ;
    freopen("ordine.out", "w", stdout) ;
    std::cin >> s ;
    register int i ;
    register char j ;
    char Ans, Previous(0) ;
    for (i = 0 ; i < s.size() ; ++ i) {
        mp[s[i]] ++ ;
    }
    bool Good ;
    for (i = 0 ; i < s.size() ; ++ i) {
        Good = true ;
        for (j = 'a' ; j <= 'z' ; ++ j) {
            if ((mp[j] > 0 && j != Previous && Good == true) || (mp[j] >= (s.size() - i) / 2 + 1 && j != Previous)) {
                Ans = j ;
                Good = false ;
            }
        }
        mp[Ans] -- ;
        Previous = Ans ;
        printf("%c", Ans) ;
    }
    return 0;
}