s=input()
l=[""]*26
for i in s:
    if((i>='a' and i<='z')):
        l[ord(i)-97]+=i
    elif(i>='A' and i<='Z'):       
        l[ord(i)-65]+=i
l1=[i for i in l if(i!='')]
print(l1)
i=0
j=len(l1)-1
ans=''
while(i<=j):
    if(i==j):
        ans+=l1[i]
    else:
        ans+=l1[i]+l1[j]
    i+=1
    j-=1
print(ans)