var globalSlideNumber = 1;

showSlide(1);

function showSlide(slideNumber){
  for(i = 1; i <= 4; i++){
    document.getElementById('slide' + i).style.display = 'none';
    if(i == 1){
      document.getElementById('btnSlide' + i).className = 'first';
    } else {
      document.getElementById('btnSlide' + i).className = '';
    }
  }
  document.getElementById('slide' + slideNumber).style.display = 'block';
  if(slideNumber == 1){
    document.getElementById('btnSlide' + slideNumber).className = 'first firstAct';
  } else {
    document.getElementById('btnSlide' + slideNumber).className = 'act';
  }
  
  if(slideNumber < 4){
    document.getElementById('btnNext').style.display = '';
  } else {
    document.getElementById('btnNext').style.display = 'none';
  }
  
  if(slideNumber > 1){
    document.getElementById('btnBack').style.display = '';
  } else {
    document.getElementById('btnBack').style.display = 'none';
  }

  globalSlideNumber = slideNumber;

  return false;
}

function nextSlide(){
  globalSlideNumber = globalSlideNumber + 1;
  if(globalSlideNumber > 4){
    globalSlideNumber = 1;
  }
  showSlide(globalSlideNumber);
}

function previousSlide(){
  globalSlideNumber = globalSlideNumber - 1;
  if(globalSlideNumber < 1){
    globalSlideNumber = 4;
  }
  showSlide(globalSlideNumber);
}