#include <bits/stdc++.h>
#define ll long long
#define str string
#define pii pair<int, int>
#define pll pair<ll, ll>
#define fi first
#define se second
#define pb push_back
#define SET(a, b) memset(a, b, sizeof(a))
#define mod 1000000007
#define inf 1000000000
#define llinf 1000000000000000000
#define eps 1e-6
#define pi atan(1) * 4
using namespace std;
vector<int> pos;
int main () {
int a, b, c, d, e, f, g, h, i, j, k, m, n, o, p, q;
str A, B, C;
ios::sync_with_stdio(false);
cin.tie(0);
cin >> A;
a = A.length();
for (i = 0; i < a; i++) {
if (A[i] != 'x') {
B += A[i];
pos.pb(i);
}
}
b = B.length();
for (i = 0; i < b; i++) {
if (B[i] != B[b - 1 - i]) {
printf("-1\n");
return 0;
}
}
int ans = 0;
int l, r;
if (b & 1) {
l = pos[b >> 1] - 1; r = pos[b >> 1] + 1;
} else {
l = pos[b >> 1]; r = pos[(b >> 1) + 1];
}
for (i = l, j = r; i >= 0; i--, j++) {
int m = 0, m2 = 0;
while (i >= 0 && A[i] == 'x') m++, i--;
while (j < A.length() && A[j] == 'x') m2++, j++;
ans += abs(m - m2);
}
ans += abs((i + 1) - (a - j));
printf("%d\n", ans);
return 0;
}