# C





S = eval(input())

N = len(S)

# 回分判定

ss = ""

for s in S:

    if s == 'x':

        pass

    else:

        ss += s

if ss != ss[::-1]:

    print((-1))

    exit()

if ss == "":

    print((0))

    exit()

# 真ん中を決める

# x の位置と個数を求める

n = len(ss)

if n % 2 == 0:

    cnt = 0

    for i in range(n//2+1):

        for j in range(cnt, N):

            if S[j] == "x":

                continue

            else:

                cnt = j+1

                break

        if i == n//2-1:

            left_center = cnt-1

    right_center = cnt-1

    # 真ん中の分を先に計算する

    ans = (abs(right_center - left_center) % 2)

    # centerから両側に展開していく

    count = 0

    x_left = []

    x_right = []

    for i in range(left_center):

        if S[left_center-1 - i] == 'x':

            count += 1

        else:

            x_left.append(count)

            count = 0

    x_left.append(count)

    count = 0

    for i in range(right_center+1, N, 1):

        if S[i] == 'x':

            count += 1

        else:

            x_right.append(count)

            count = 0

    x_right.append(count)

    ans = 0

    for i in range(len(x_left)):

        ans += abs(x_left[i] - x_right[i])

    print(ans)

else:

    cnt = 0

    for i in range(n//2+1):

        for j in range(cnt, N):

            if S[j] == "x":

                continue

            else:       # S[j] != "x" なら ss[i] と S[j] は一致する

                cnt = j + 1

                break

    center = cnt-1

    # print(center)

    # center から両側に展開していき，center からの位置と x の個数を求める

    distans = 0     # x 以外の文字が出るたびに更新する

    count = 0

    x_left = []

    x_right = []

    for i in range(center):

        if S[center-1 - i] == "x":

            count += 1

        else:

            x_left.append([distans, count])

            distans += 1

            count = 0

    x_left.append([distans, count])

    distans = 0

    count = 0

    for i in range(center+1, N):

        if S[i] == "x":

            count += 1

        else:

            x_right.append([distans, count])

            distans += 1

            count = 0

    x_right.append([distans, count])

    # print(x_left)

    # print(x_right)

    ans = 0

    for i in range(len(x_left)):

        ans += abs(x_left[i][1] - x_right[i][1])

    print(ans)

S = eval(input())

N = len(S)

r = 0

l = 0

ans = 0

while l+r < N:

    if S[l] == S[N-1 - r]:

        l += 1

        r += 1

    else:

        if S[l] == 'x':

            l += 1

            ans += 1

        elif S[N-1 - r] == 'x':

            r += 1

            ans += 1

        else:

            print((-1))

            exit()



print(ans)
# C
S = eval(input())
N = len(S)
# 回分判定
ss = ""
for s in S:
    if s == "x":
        pass
    else:
        ss += s
if ss != ss[::-1]:
    print((-1))
    exit()
if ss == "":
    print((0))
    exit()
# 真ん中を決める
# x の位置と個数を求める
n = len(ss)
if n % 2 == 0:
    cnt = 0
    for i in range(n // 2 + 1):
        for j in range(cnt, N):
            if S[j] == "x":
                continue
            else:
                cnt = j + 1
                break
        if i == n // 2 - 1:
            left_center = cnt - 1
    right_center = cnt - 1
    # 真ん中の分を先に計算する
    ans = abs(right_center - left_center) % 2
    # centerから両側に展開していく
    count = 0
    x_left = []
    x_right = []
    for i in range(left_center):
        if S[left_center - 1 - i] == "x":
            count += 1
        else:
            x_left.append(count)
            count = 0
    x_left.append(count)
    count = 0
    for i in range(right_center + 1, N, 1):
        if S[i] == "x":
            count += 1
        else:
            x_right.append(count)
            count = 0
    x_right.append(count)
    ans = 0
    for i in range(len(x_left)):
        ans += abs(x_left[i] - x_right[i])
    print(ans)
else:
    cnt = 0
    for i in range(n // 2 + 1):
        for j in range(cnt, N):
            if S[j] == "x":
                continue
            else:  # S[j] != "x" なら ss[i] と S[j] は一致する
                cnt = j + 1
                break
    center = cnt - 1
    # print(center)
    # center から両側に展開していき，center からの位置と x の個数を求める
    distans = 0  # x 以外の文字が出るたびに更新する
    count = 0
    x_left = []
    x_right = []
    for i in range(center):
        if S[center - 1 - i] == "x":
            count += 1
        else:
            x_left.append([distans, count])
            distans += 1
            count = 0
    x_left.append([distans, count])
    distans = 0
    count = 0
    for i in range(center + 1, N):
        if S[i] == "x":
            count += 1
        else:
            x_right.append([distans, count])
            distans += 1
            count = 0
    x_right.append([distans, count])
    # print(x_left)
    # print(x_right)
    ans = 0
    for i in range(len(x_left)):
        ans += abs(x_left[i][1] - x_right[i][1])
    print(ans)

S = eval(input())
N = len(S)
r = 0
l = 0
ans = 0
while l + r < N:
    if S[l] == S[N - 1 - r]:
        l += 1
        r += 1
    else:
        if S[l] == "x":
            l += 1
            ans += 1
        elif S[N - 1 - r] == "x":
            r += 1
            ans += 1
        else:
            print((-1))
            exit()
print(ans)

-# C -# 回分判定 -ss = "" -for s in S: -    if s == "x": -        pass +r = 0 +l = 0 +ans = 0 +while l + r < N: +    if S[l] == S[N - 1 - r]: +        l += 1 +        r += 1 -        ss += s -if ss != ss[::-1]: -    print((-1)) -    exit() -if ss == "": -    print((0)) -    exit() -# 真ん中を決める -# x の位置と個数を求める -n = len(ss) -if n % 2 == 0: -    cnt = 0 -    for i in range(n // 2 + 1): -        for j in range(cnt, N): -            if S[j] == "x": -                continue -            else: -                cnt = j + 1 -                break -        if i == n // 2 - 1: -            left_center = cnt - 1 -    right_center = cnt - 1 -    # 真ん中の分を先に計算する -    ans = abs(right_center - left_center) % 2 -    # centerから両側に展開していく -    count = 0 -    x_left = [] -    x_right = [] -    for i in range(left_center): -        if S[left_center - 1 - i] == "x": -            count += 1 +        if S[l] == "x": +            l += 1 +            ans += 1 +        elif S[N - 1 - r] == "x": +            r += 1 +            ans += 1 -            x_left.append(count) -            count = 0 -    x_left.append(count) -    count = 0 -    for i in range(right_center + 1, N, 1): -        if S[i] == "x": -            count += 1 -        else: -            x_right.append(count) -            count = 0 -    x_right.append(count) -    ans = 0 -    for i in range(len(x_left)): -        ans += abs(x_left[i] - x_right[i]) -    print(ans) -else: -    cnt = 0 -    for i in range(n // 2 + 1): -        for j in range(cnt, N): -            if S[j] == "x": -                continue -            else:  # S[j] != "x" なら ss[i] と S[j] は一致する -                cnt = j + 1 -                break -    center = cnt - 1 -    # print(center) -    # center から両側に展開していき，center からの位置と x の個数を求める -    distans = 0  # x 以外の文字が出るたびに更新する -    count = 0 -    x_left = [] -    x_right = [] -    for i in range(center): -        if S[center - 1 - i] == "x": -            count += 1 -        else: -            x_left.append([distans, count]) -            distans += 1 -            count = 0 -    x_left.append([distans, count]) -    distans = 0 -    count = 0 -    for i in range(center + 1, N): -        if S[i] == "x": -            count += 1 -        else: -            x_right.append([distans, count]) -            distans += 1 -            count = 0 -    x_right.append([distans, count]) -    # print(x_left) -    # print(x_right) -    ans = 0 -    for i in range(len(x_left)): -        ans += abs(x_left[i][1] - x_right[i][1]) -    print(ans) +            print((-1)) +            exit() +print(ans)