1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
S = list(input().strip())

n = len(S)


def quick_test(S):
    n = len(S)
    A = sum(x == 'a' for x in S)
    B = n - A
    if n % 2 == 0:
        return A % 2 == 0 and B % 2 == 0
    else:
        return (A % 2 == 0) ^ (B % 2 == 0)


def aux(S, a, b):
    acc = 0
    i = a
    j = b
    while True:
        if a == b:
            return acc
        if a + 1 == b:
            return acc + (S[a] != S[b])

        if S[a] == S[b]:
            a = a+1
            b = b-1
            continue

        x = S[a]
        for i in range(max(a, i), b+1):
            if S[i] != x:
                break

        x = S[b]
        for j in range(min(b, j), a-1, -1):
            if S[j] != x:
                break

        if (i - a) < (b - j):
            # A ma blizej
            S[a], S[i] = S[i], S[a]
        else:
            S[b], S[j] = S[j], S[b]

        acc += min(i-a, b-j)
        a = a+1
        b = b-1


def main(S):
    return aux(S, 0, len(S) - 1)


if not quick_test(S):
    print("-1")
else:
    print(main(S))