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s = list(input())
n = len(s)

z = []
o = []
for i in range(n): z.append(i) if s[i] == 'a' else o.append(i)

c=0
for i in range(n//2):
    if s[i] != s[n-i-1]:
        a = o[0]-i if s[i] == 'a' else z[0] -i
        b = n-1-i-o[-1] if s[n-i-1] == 'a' else n-i-1-z[-1]
        if a<b:
            if s[i] == 'a':
                s[z[0]], s[o[0]]=s[o[0]], s[z[0]]
                z[0]=o[0]
                o.pop(0)
            else:
                s[o[0]], s[z[0]]=s[z[0]], s[o[0]]
                o[0] = z[0]
                z.pop(0)
            if s[n-i-1] == 'a':
                z.pop()
            else:
                o.pop()
            c+=a
        else:
            if s[n-i-1] == 'b':
                s[o[-1]], s[z[-1]]=s[z[-1]], s[o[-1]]
                o[-1]=z[-1]
                z.pop()
            else:
                s[z[-1]], s[o[-1]]=s[o[-1]], s[z[-1]]
                z[-1]=o[-1]
                o.pop()

            if s[i] == 'a':
                z.pop(0)
            else:
                o.pop(0)
            c+=b
    else:
        if s[i] == 'a':
            z.pop(0)
        else:
            o.pop(0)
        if s[n-i-1] == 'a':
            z.pop()
        else:
            o.pop()
ok = 1
for i in range(n//2):
    if s[i] != s[n-i-1]:
        ok =0
        break
print(c) if ok == 1 else print(-1)