Edit in JSFiddle

var boton = document.getElementById("s"),
    a = document.getElementById("a"),
    b = document.getElementById("b"),
    c = document.getElementById("c");

s.addEventListener("click", function(){
    if (!isNaN(b.value))
        c.value = Number(a.value) + Number(b.value);
}, false);

Cantidad 1: <input type = "text" id = "a" value = 5 readOnly />
Cantidad 2: <input type = "text" id = "b" autofocus />
Resultado: <input type = "text" id = "c" readOnly />
<button id = "s">Sumar</button>