function counter( string ) {
  var maxLength = 100;
  var oSpan = document.getElementById( 'counter' );
  oSpan.innerHTML = 'Pozostało Ci '+ (maxLength - ( string.length ) ) +' znaków';
  if( maxLength - ( string.length ) < -1 )
    document.getElementById( 'oText' ).disabled=true;
}