// eddy1021
#pragma GCC optimize("O3")
#include <bits/stdc++.h>
using namespace std;
typedef double D;
typedef long double LD;
typedef long long LL;
typedef pair<int,int> PII;
typedef pair<LL,LL> PLL;
#define mod9 1000000009LL
#define mod7 1000000007LL
#define INF  1023456789LL
#define INF16 10000000000000000LL
#define eps 1e-9
#define SZ(x) (int)(x).size()
#define ALL(x) (x).begin(), (x).end()
#define IOS ios_base::sync_with_stdio(0); cin.tie(0)
#ifndef ONLINE_JUDGE
#define debug(...) printf(__VA_ARGS__)
#else 
#define debug(...)
#endif
inline LL getint(){
  LL _x=0,_tmp=1; char _tc=getchar();    
  while( (_tc<'0'||_tc>'9')&&_tc!='-' ) _tc=getchar();
  if( _tc == '-' ) _tc=getchar() , _tmp = -1;
  while(_tc>='0'&&_tc<='9') _x*=10,_x+=(_tc-'0'),_tc=getchar();
  return _x*_tmp;
}
inline LL add( LL _x , LL _y , LL _mod = mod7 ){
  _x += _y;
  return _x >= _mod ? _x - _mod : _x;
}
inline LL sub( LL _x , LL _y , LL _mod = mod7 ){
  _x -= _y;
  return _x < 0 ? _x + _mod : _x;
}
inline LL mul( LL _x , LL _y , LL _mod = mod7 ){
  _x *= _y;
  return _x >= _mod ? _x % _mod : _x;
}
LL mypow( LL _a , LL _x , LL _mod ){
  if( _x == 0 ) return 1LL;
  LL _ret = mypow( mul( _a , _a , _mod ) , _x >> 1 , _mod );
  if( _x & 1 ) _ret = mul( _ret , _a , _mod );
  return _ret;
}
LL mymul( LL _a , LL _x , LL _mod ){
  if( _x == 0 ) return 0LL;
  LL _ret = mymul( add( _a , _a , _mod ) , _x >> 1 , _mod );
  if( _x & 1 ) _ret = add( _ret , _a , _mod );
  return _ret;
}
inline bool equal( D _x ,  D _y ){
  return _x > _y - eps && _x < _y + eps;
}
void sleep( double sec = 1021 ){
  clock_t s = clock();
  while( clock() - s < CLOCKS_PER_SEC * sec );
}
#define Bye exit(0)
int __ = 1 , _cs;
/*********default*********/
#define N 101010
void build(){

}
char s[ N ] , t[ N ];
int len , tlen;
void init(){
  cin >> s;
  len = strlen( s );
}
void solve(){
  for( int i = 0 ; i < len ; i ++ )
    if( s[ i ] != 'x' )
      t[ tlen ++ ] = s[ i ];
  if( tlen == 0 ){
    puts( "0" );
    Bye;
  }
  for( int i = 0 ; i < tlen ; i ++ )
    if( t[ i ] != t[ tlen - i - 1 ] ){
      puts( "-1" );
      Bye;
    }
  int ans = 0;
  if( tlen & 1 ){
    int who = 0 , cnt = 0;
    for( int i = 0 ; i < len ; i ++ )
      if( s[ i ] != 'x' ){
        cnt ++;
        if( cnt + cnt == tlen + 1 ){
          who = i;
          break;
        }
      }
    int lptr = who - 1 , rptr = who + 1;
    while( lptr >= 0 or rptr < len ){
      int lc = 0 , rc = 0;
      while( lptr >= 0 and s[ lptr ] == 'x' ){
        lc ++;
        lptr --;
      }
      lptr --;
      while( rptr < len and s[ rptr ] == 'x' ){
        rc ++;
        rptr ++;
      }
      rptr ++;
      ans += abs( lc - rc );
    }
  }else{
    int lwho = 0 , rwho = 0 , cnt = 0;
    for( int i = 0 ; i < len ; i ++ )
      if( s[ i ] != 'x' ){
        cnt ++;
        if( cnt + cnt == tlen )
          lwho = i;
        if( ( cnt - 1 ) * 2 == tlen )
          rwho = i;
      }
    int lptr = lwho - 1 , rptr = rwho + 1;
    while( lptr >= 0 or rptr < len ){
      int lc = 0 , rc = 0;
      while( lptr >= 0 and s[ lptr ] == 'x' ){
        lc ++;
        lptr --;
      }
      lptr --;
      while( rptr < len and s[ rptr ] == 'x' ){
        rc ++;
        rptr ++;
      }
      rptr ++;
      ans += abs( lc - rc );
    }
  }
  cout << ans << endl;
}
int main(){
  build();
  //__ = getint();
  while( __ -- ){
    init();
    solve();
  }
}


Measured memory_bytes: 384