JSFiddle - React, Tailwind, and code Playground

HTML

<p>Число 1: <input type="text" id="n1"></p>
         <p>Число 2: <input type="text" id="n2"></p>
                <button onclick="plus()">Сложить</button>
                <button>Вычесть</button>
                
         <hr width="35%" align="left">
                
         <p id="out">Результат:</p>

JavaScript

function plus(){
        var num1, num2, result;
        num1 = document.getElementById("n1").value;
        num1 = parseInt(num1);
        
        num2 = document.getElementById("n2").value;
        num2 = parseInt(num2);
        
        result = num1 + num2;
        document.getElementById("out").innerHTML = result;
}