function overeni()
{
  // promenne:
  var jmeno = document.getElementById('jmeno');
  var email = document.getElementById('email');
  var predmet = document.getElementById('predmet');
  var zneni = document.getElementById('zneni');
  
  var jmeno_pov = document.getElementById('jmeno_pov');
  var email_pov = document.getElementById('email_pov');
  var email_nepl = document.getElementById('email_nepl');
  var predmet_pov = document.getElementById('predmet_pov');
  var zneni_pov = document.getElementById('zneni_pov');
  
  var ok = true;
  
  jmeno_pov.style.display = 'none';
  email_pov.style.display = 'none';
  email_nepl.style.display = 'none';
  predmet_pov.style.display = 'none';
  zneni_pov.style.display = 'none';
  
  // overovani:
  if (jmeno.value == "")
  {
    jmeno_pov.style.display = 'inline';
    ok = false;
  }
  
  var posDot = (email.value).lastIndexOf(".");
  if (posDot == -1)
    posDot = 0;
  var strAfterDot = (email.value).substr(posDot + 1);
  if (email.value == "" || email.value == "@")
  {
    email_pov.style.display = 'inline';
    ok = false;
  }
  else if ((email.value).indexOf("@") == -1 || (email.value).indexOf(".") == -1 || (email.value).length < 6 || 
            strAfterDot.length < 2 || strAfterDot.indexOf("@") > -1)
  {
    email_nepl.style.display = 'inline';
    ok = false;
  }
  
  if (predmet.value == "")
  {
    predmet_pov.style.display = 'inline';
    ok = false;
  }
  
  if (zneni.value == "")
  {
    zneni_pov.style.display = 'inline';
    ok = false;
  }
  
  // vraceni vysledku funkce:
  return ok;
}