1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
import sys
s=input()
if (s.count("a") == 0 or  s.count("b")  == 0):
    print(0)
    sys.exit(0)


if (s.count("a") % 2 == 1 and  s.count("b") % 2 == 1):
    print(-1)
    sys.exit(0)
i,j=0,len(s)-1
ev="a"
if (s.count("b") % 2 == 0): ev = "b"

L=j
res=0
while (i<j):
    while(i < j and s[i]!=ev): i += 1
    while(j > i and s[j]!=ev): j -= 1
    res += abs(i - (L - j))
    i += 1
    j -= 1

print(res)