var a = document.getElementById("a"), b = document.getElementById("b"), c = document.getElementById("c"); b.addEventListener("keyup", function(){ if (!isNaN(this.value)) c.value = Number(a.value) + Number(this.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 />