def solution(s):
d = dict()
for ch in s:
if ch in d:
d[ch] += 1
else:
d[ch] = 1
odd_letters = []
for ch in d:
if d[ch] % 2 != 0:
odd_letters.append(ch)
if len(odd_letters) == 0:
pass
else:
odd_letters.sort(key=lambda ch : ord(ch))
n = len(odd_letters)
for i in range(n//2):
d[odd_letters[i]] += 1
d[odd_letters[n-i-1]] -= 1
parts = []
for ch in sorted(d.keys(), key=lambda ch: ord(ch)):
parts.append(ch * (d[ch]//2))
rparts = reversed(parts)
for ch in d.keys():
if d[ch] % 2 != 0:
parts.append(ch)
break
parts.extend(rparts)
return ''.join(parts)
def test():
pass
def main():
test()
s = input()
print(solution(s))
main()
#include<bits/stdc++.h>
using namespace std;
typedef long long int ll;
#define pi(x) cout<<x;
#define ps(x) cout<<x<<" ";
#define pnl(x) cout<<x<<"\n";
#define for0(n) for(i=0;i<n;i++)
#define for1(n) for(i=1;i<=n;i++)
#define m(x) memset(x,0,sizeof x);
#define nl cout<<"\n";
#define mp make_pair
#define pb push_back
#define fr first
#define se second
#define Inf 1e16
#define MOD 1e9+7
vector<ll>a[100000];
ll dp[100000];
ll xr[100000];
ll countx=0,xx=0;
void dfs(ll x,ll p){
xr[x]=dp[x];
for(auto dd:a[x]){
if(p!=dd){
dfs(dd,x);
xr[x]^=xr[dd];
}
}
if(xr[x]==xx){
xr[x]=0;
countx+=(p!=0);
}
}
int main(){
ios_base::sync_with_stdio(false);
cin.tie(NULL);
cout.tie(NULL);
ll test,h,p,i,j,xy,flag=0,n,u,count,d,o1=0,o2=0,s,e,l,r,x,y,m,z,max1,x1,y1,k,x2,y2,z1,z2,sum,min1;
string a;
cin>>a;
ll dp[26];
ll od[26];
memset(dp,0,sizeof dp);
memset(od,0,sizeof od);
for(i=0;i<a.size();i++){
dp[a[i]-'a']++;
od[a[i]-'a']++;
od[a[i]-'a']%=2;
}
for(i=0;i<26;i++){
if(od[i]){
for(j=25;j>i;j--){
if(od[j]){
dp[i]++;
od[i]--;
od[j]--;
break;
}
}
}
}
n=a.size();
count=0;
for(i=0;i<n-i-1;i++){
while(count<26&&dp[count]<2){
count++;
}
if(count>=26){
break;
}
a[i]=(count+'a');
a[n-i-1]=(count+'a');
dp[count]-=2;
}
if(n%2){
for(i=0;i<26;i++){
if(od[i]){
a[n/2]=(i+'a');
}
}
}
cout<<a<<"\n";
return 0;
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