B

from math import ceil
t = input()
s = {}
for i in t:
    if i in s.keys():
        s[i] +=1
    else:
        s[i] = 1
k = list(list(s.items()))
k.sort(key= lambda i: i[1])
if k[len(k)-1][1]>ceil(len(t)/2):
    print("-1")
else:
    tt = k[-1][0] * k[-1][1]
    for i in range(len(k)-1):
        tt = tt+ k[i][0] * k[i][1]
    ss =""
    nn = len(tt)
    if nn%2==0:
        for i in range(nn//2):
            ss =ss+tt[i]+tt[nn//2+i]
    else:
        for i in range(nn//2):
            ss =ss+tt[i]+tt[nn//2+i+1]
        ss = ss+tt[nn//2]
    print(ss)