a = input()[:-1]
dop = 0
string = ''
slov = {}
for i in a:
    slov[i] = slov.get(i, 0) + 1
for i in slov:
    if slov[i] % 2 == 0:
        continue
    elif slov[i] % 2 == 1:
        string += slov[i]
        dop += 1
if dop > 1:
    print('NO')
else:
    for i in range(1, len(sorted(slov)) + 1):
        if len(slov) // 2 == i:
            print(string, end='')
        elif slov[i] % 2 == 0:
            print(slov[i], end='')