#include <bits/stdc++.h>
#include <iostream>
#define ll long long int 
using namespace std;
typedef unsigned long long ull;
#define M_PI 3.14159265358979323846
#pragma GCC optimize ("Ofast")
#pragma GCC target("fma,sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,avx2,tune=native")
#pragma GCC optimize("unroll-loops")
#define nl "\n"
#define sp " "
 
 
int main()
{
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);cout.tie(NULL);
    
    // freopen("input.txt","r",stdin);
    // freopen("output.txt","w",stdout);
    
    string s;cin>>s;
    unordered_map<char,int> mp;
 
    for(char ch:s)
    {
        mp[ch]++;
    }
    int c=0;char ch='$';
    for(auto &x:mp)
    {
        if(x.second%2==0){continue;}
        else{c++;ch=x.first;}
        if(c>1){break;}
    }
    if(c>1){
        cout<<"NO SOLUTION"<<endl;return 0;
    }
 
    
    if(c==1)
    {
        string ans(mp[ch],ch);
        for(auto &x:mp)
        {
            if(x.first!=ch)
            {
                string k(x.second/2,x.first);
                ans=k+ans+k;
            }
        }
        cout<<ans<<endl;
    }
    else{
        string ans="";
        for(auto &x:mp)
        {
            string k(x.second/2,x.first);
            ans=k+ans+k;
        }
        cout<<ans<<endl;
    }
 
}