Работа с input и button

    function reset() { 
        var res1 = document.getElementById('out'); 
        var res2 = document.getElementById('n1'); 
        var res3 = document.getElementById('n2'); 
        res1.value = ''; 
        res2.value = ''; 
        res3.value = ''; 
    } 
     
     
    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').value = result; 
         
    } 
     
    function minus() { 
        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').value = result; 
         
    } 
     
    function multiply() { 
        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').value = result; 
         
    } 
     
    function divide() { 
        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').value = result; 
         
    } 
     
    function numBr() { 
         
    } 
     
    function onFocus() { 
         
    }

        <div id='first'> 
    <hr>  
        <input type="text" id='n1' value='' placeholder='First number...' onclick='onFocus()'> 
        <input type="text" id='n2' value='' placeholder='Second number' onclick='onFocus1'> 
    <hr> 
    <br> 
        <button onclick="plus()">Plus +</button> 
        <button onclick="minus()">Minus -</button> 
        <button onclick="multiply()">Multiply *</button> 
        <button onclick="divide()">Divide /</button> 
        <button onclick="reset()">Clear</button> 
    <hr> 
    <br> 
        <input type='text' id='out' value='' disabled='true' placeholder='Output...'> 
    <hr> 
    <br> 
        <input type='button' onclick='numBr()' value='0'> 
        <input type='button' onclick='numBr()' value='1'> 
        <input type='button' onclick='numBr()' value='2'> 
        <input type='button' onclick='numBr()' value='3'> 
        <input type='button' onclick='numBr()' value='4'> 
        <input type='button' onclick='numBr()' value='5'> 
        <input type='button' onclick='numBr()' value='6'> 
        <input type='button' onclick='numBr()' value='7'> 
        <input type='button' onclick='numBr()' value='8'> 
        <input type='button' onclick='numBr()' value='9'> 
    <hr>      
        </div>