//
import java.math.*;
import java.util.*;
import java.io.*;
  
public class Main { 
  
    static BufferedReader in;
    static PrintWriter out = new PrintWriter(System.out);
    static String file = "../in";
    static int test = 10; // 0 for local testing, 1 for std input
    static int inf = 1_000_000, mod = 1_000_000_007;

    void swap(int[]ary, int i, int j)
    {
        int t = ary[i];
        ary[i] = ary[j];
        ary[j] = t;
    }
    
    String[] split() throws Exception
    {
        return in.readLine().split(" ");
    }

    int readInt() throws Exception
    {
        return Integer.valueOf(in.readLine());
    }

    int[] toIntArray() throws Exception
    {
        String[] sp = split();
        int n = sp.length;
        int[] ary = new int[n];
        for(int i = 0; i < n; i++) ary[i] = Integer.valueOf(sp[i]);
        return ary;
    }

    String reverse(String str)
    {
        return new StringBuilder(str).reverse().toString();
    }

    long choose(int n, int a)
    {
        long nf = 1;
        for(int i = 1; i <= a; i++)
        {
            nf *= i;
            nf %= mod;
            nf *= pow(i, mod - 2);
            nf %= mod;
        }
        return nf;
    }

    long pow(int a, int pow)
    {
        return pow(0L + a, pow);
    }
    
    long pow(long a, int pow)
    {
        long res = 1;
        while(pow > 0)
        {
            if(pow % 2 != 0) {
                res *= a;
                res %= mod;
            }
            a = a * a;
            a %= mod;
            pow /= 2;
        }

        return res;
    }    

    public static void main(String[] args) throws Exception
    {
        int _k = Integer.valueOf("1");
        if(test > 0) in = new BufferedReader(new InputStreamReader(System.in));
        else in = new BufferedReader(new FileReader(file));
        if(test < 0) {String[] str = in.readLine().split(" ");}
/***********************************************************************/
/***********************************************************************/
/***********************************************************************/
/***********************************************************************/
        // System.out.println((-100 + 0) / 2);

        new Main().c();
        out.flush();
    }

    void c() throws Exception
    {
        String str = in.readLine();
        // if str without x is palindrome
        // then inserting can make it palin
        // StringBuilder sb = new StringBuilder();

        // for(char c : str.toCharArray())
        // {
        //     if(c != 'x') sb.append(c);
        // }

        // if(!isPalindrome(sb.toString()))
        // {
        //     System.out.println(-1);
        //     return;
        // }

        int i = 0, j = str.length() - 1, op = 0;
        while(i < j)
        {
            if(str.charAt(i) == str.charAt(j)) 
            {
                i++;
                j--;
            }
            else
            {
                if(str.charAt(i) == 'x')
                {
                    i++;
                    op++;
                }
                else if(str.charAt(j) == 'x')
                {
                    j--;
                    op++;
                }
                else
                {
                    System.out.println(-1);
                    return;
                }
            }
        }
        out.println(op);
    }

    boolean isPalindrome(String str)
    {
        int i = 0, j = str.length() - 1;
        while(i < j)
        {
            if(str.charAt(i) != str.charAt(j)) return false;
            i++;
            j--;
        }
        return true;
    }

    int find(int[] f , int p)
    {
        int fp = f[p];
        while(fp != f[fp]) fp = f[f[fp]];
        return f[p] = fp;
    }
}


Measured memory_bytes: 24788