Shared code

/* 777 */
#include "bits/stdc++.h"
using namespace std;

#define int long long 

// const int MOD=1e9+7;

void solve(){
	string s;cin>>s;
	int freq[26]={0};
	for(auto &c:s)freq[c-'A']++;
	string res;
	int oddcnt=0,oddind=-1;
	for(int i=0;i<26;++i){
		if(freq[i]&1){
			oddcnt++;
			oddind=i;
		}
		for(int f=0;f<freq[i]/2;++f)res.push_back('A'+i);
	}
	if(oddcnt>1){
		cout<<"NO SOLUTION";
		return;
	}
	cout<<((oddind!=-1)?(res+char('A'+oddind)+string(res.rbegin(),res.rend())):res+string(res.rbegin(),res.rend()));
}

signed main(){
	ios::sync_with_stdio(false);
	cin.tie(nullptr);
	solve();
}