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 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 import sys def madrak(s0): d0 = len(s0) an = s0.count('a') bn = s0.count('b') if an == d0 or bn == d0: return 0 if d0 % 2 == 0 and an % 2 == 1: return -1 s = list(s0) i = 0 j = d0 - 1 dm = 0 a = 0 while s[a] != 'a': a += 1 b = 0 while s[b] != 'b': b += 1 c = j while s[c] != 'a': c -= 1 d = j while s[d] != 'b': d -= 1 # print(f'{a} {b} {c} {d}') while i < j: # print(f'i={i} j={j} a={a} b={b} c={c} d={d}') if s[i] == 'a' and s[j] == 'a': a += 1 while a < j and s[a] != 'a': a += 1 if a >= j: return dm c -= 1 while c > i and s[c] != 'a': c -= 1 if c <= i: return dm elif s[i] == 'b' and s[j] == 'b': b += 1 while b < j and s[b] != 'b': b += 1 if b >= j: return dm d -= 1 while d > i and s[d] != 'b': d -= 1 if d <= i: return dm elif s[i] == 'a' and s[j] == 'b': if abs(a-b) <= abs(c-d): dm += abs(a-b) s[b] = 'a' if c == b-1: c = b a += 1 b += 1 while b < j and s[b] != 'b': b += 1 if b >= j: return dm d -= 1 while d > i and s[d] != 'b': d -= 1 if d <= i: return dm else: dm += abs(c-d) s[c] = 'b' if b == c+1: b = c d -= 1 c -= 1 while c > i and s[c] != 'a': c -= 1 if c <= i: return dm a += 1 while a < j and s[a] != 'a': a += 1 if a >= j: return dm elif s[i] == 'b' and s[j] == 'a': if abs(a-b) <= abs(c-d): dm += abs(a-b) s[a] = 'b' if d == a-1: d = a b += 1 a += 1 while a < j and s[a] != 'a': a += 1 if a >= j: return dm c -= 1 while c > i and s[c] != 'a': c -= 1 if c <= i: return dm else: dm += abs(c-d) s[d] = 'a' if a == d+1: a = d c -= 1 d -= 1 while d > i and s[d] != 'b': d -= 1 if d <= i: return dm b += 1 while b < j and s[b] != 'b': b += 1 if b >= j: return dm i += 1 j -= 1 return dm s = input().strip() print(madrak(s))