JSFiddle - React, Tailwind, and code Playground

HTML

<p class="cont" id="res">
    A soma dos primeiros 
    <input id="cont" type="number" name="num" min="0" max="100000000" />
    multiplos de 
    <input id="mul1" type="number" name="num" min="1" max="1000" /> 
   e <input id="mul2" type="number" name="num" min="1" max="1000" /> Ă©:
    <span id="resultado"></span>
</p>
<br />
<input type="button" id="calcmult" value="Calcular" />

JavaScript

//quando a janela carrega eu salvo o estado inicial do <p>
    var pad = document.getElementById("res");
    //funcao q faz o calculo
    document.getElementById("calcmult").onclick = function () {

        var contador = document.getElementById("cont").value;
        var num1 = document.getElementById("mul1").value;
        var num2 = document.getElementById("mul2").value;
        var i=0;
        var x=0;
        var y=0;
        while(i<=contador){
            if(x%num1 == 0 && x%num2 == 0){
                y = y + x;
                i++;
            }
            x++;
        }
        
        x = document.getElementById("resultado").innerText = y;
        
        //zero os valores que estavam nos campos
        document.getElementById("cont").value = '';
        document.getElementById("mul1").value = '';
        document.getElementById("mul2").value = '';
    }