import java.io.InputStream;
import java.io.IOException;
import java.io.PrintStream;
import java.util.*;

public class Main {

  public static void main(String[] args) {
    solve(System.in, System.out);
  }

  static void solve(InputStream is, PrintStream os) {
    Scanner sc = new Scanner(is);

    /* read */
    String s = sc.next();

    String removed = s.replace("x", "");

    int length = (removed.length() + 1) / 2;

    // check
    int l = 0;
    int r = removed.length() - 1;
    for (int i = 0; i < length; i++) {
      if (removed.charAt(l + i) != removed.charAt(r - i)) {
        os.println("-1");
        return;
      }
    }

    l = -1;
    r = s.length();
    int count = 0;
    while (count < length) {
      l++;
      if (s.charAt(l) != 'x') count++;
    }
    count = 0;
    while (count < length) {
      r--;
      if (s.charAt(r) != 'x') count++;
    }

    int ans = 0;
    while (0 <= l && r < s.length()) {
      char lc = s.charAt(l);
      char rc = s.charAt(r);
      if (lc == rc) {
        l--;
        r++;
      } else if (lc == 'x') {
        // insert x to right
        l--;
        ans++;
      } else if (rc == 'x') {
        // insert x to left
        r++;
        ans++;
      } else {
        os.println("-1");
        return;
      }
    }

    ans += (s.length() - r) + (l - (-1));

    os.println(ans);
  }

  private static class Scanner {

    private final InputStream is;
    private final byte[] buffer = new byte[1024];
    private int ptr = 0;
    private int buflen = 0;

    Scanner(InputStream is) {
      this.is = is;
    }

    private boolean hasNextByte() {
      if (ptr < buflen) {
        return true;
      } else {
        ptr = 0;
        try {
          buflen = is.read(buffer);
        } catch (IOException e) {
          e.printStackTrace();
        }
        if (buflen <= 0) {
          return false;
        }
      }
      return true;
    }

    private int readByte() {
      if (hasNextByte()) {
        return buffer[ptr++];
      } else {
        return -1;
      }
    }

    private static boolean isPrintableChar(int c) {
      return 33 <= c && c <= 126;
    }

    public boolean hasNext() {
      while (hasNextByte() && !isPrintableChar(buffer[ptr])) {
        ptr++;
      }
      return hasNextByte();
    }

    public String next() {
      if (!hasNext()) {
        throw new NoSuchElementException();
      }
      StringBuilder sb = new StringBuilder();
      int b = readByte();
      while (isPrintableChar(b)) {
        sb.appendCodePoint(b);
        b = readByte();
      }
      return sb.toString();
    }

    public long nextLong() {
      if (!hasNext()) {
        throw new NoSuchElementException();
      }
      long n = 0;
      boolean minus = false;
      int b = readByte();
      if (b == '-') {
        minus = true;
        b = readByte();
      }
      if (b < '0' || '9' < b) {
        throw new NumberFormatException();
      }
      while (true) {
        if ('0' <= b && b <= '9') {
          n *= 10;
          n += b - '0';
        } else if (b == -1 || !isPrintableChar(b)) {
          return minus ? -n : n;
        } else {
          throw new NumberFormatException();
        }
        b = readByte();
      }
    }

    public int nextInt() {
      long nl = nextLong();
      if (nl < Integer.MIN_VALUE || nl > Integer.MAX_VALUE) {
        throw new NumberFormatException();
      }
      return (int) nl;
    }

    public double nextDouble() {
      return Double.parseDouble(next());
    }
  }
}


Measured memory_bytes: 31512