function zt_vypocitej(formular)
  {
    var re = new RegExp("\\d+");    
    var res = 0;
    if (re.test(formular.zt_plozatepl.value) == true && re.test(formular.zt_plosotv.value) == true) {
      var a = new Number(formular.zt_plozatepl.value);
      var b = new Number(formular.zt_plosotv.value);
      res = a - b;
    }
    formular.zt_celkem.value = res;
  }

function fs_vypocitej(formular)
  {
    var re = new RegExp("\\d+");    
    var res = 0;
    if (re.test(formular.fs_ploplast.value) == true && re.test(formular.fs_plostavotv.value) == true) {
      var a = new Number(formular.fs_ploplast.value);
      var b = new Number(formular.fs_plostavotv.value);
      res = a - b;
    }
    formular.fs_celkem.value = res;
  }