join sorted list input split upper replace key lambda ord reverse True

n = "".join(sorted(list(input().split('.')[0].upper().replace(' ','')),key = lambda x: ord(x),reverse = True))
ans = ''
ans2 = ''
p = 0
while n != '':
    count = n.count(n[0])
    if count%2 == 0:
        ans += n[0]*(count//2)
        ans2 = n[0]*(count//2)+ans2
    else:
        p += 1
        t = n[0]*count
    if p == 2:
        print("NO")
        break
    n = n.replace(n[0], '')
else:
    print(ans+t+ans2)