function licz() {
    var p1 = document.getElementById("p1").value;
    var p2 = document.getElementById("p2").value;
    var p3 = document.getElementById("p3").value;
    if ( (p1.length<1) || (p2.length<1) || (p3.length<1) || isNaN(p1) || isNaN(p2) || isNaN(p3) )
        {
            return document.getElementById('komunikat').innerHTML="wpisz poprawne dane";
        }
    p1=parseFloat(p1);p2=parseFloat(p2);p3=parseFloat(p3);
    var srednia=(p1+p2+p3)/3
    return document.getElementById("wynik").innerHTML=srednia;

}