<!DOCTYPE html>
<html>
<head>
    <title>Simple Calculator</title>

    <style>
        body {
            font-family: Arial, sans-serif;
            background-color: #f2f2f2;
        }

        .calculator {
            width: 300px;
            margin: 80px auto;
            padding: 20px;
            background: white;
            border: 2px solid black;
            border-radius: 10px;
        }

        h2 {
            text-align: center;
            color: #333;
        }

        input, select, button {
            width: 90%;
            padding: 10px;
            margin: 8px;
            font-size: 16px;
        }

        #result {
            margin-top: 15px;
            font-weight: bold;
            color: blue;
        }
    </style>
</head>

<body>

    <div class="calculator">

        <h2>Simple Calculator</h2>

        <input type="number" id="num1"
               placeholder="Enter first operand">

        <select id="operator">
            <option value="+">Addition (+)</option>
            <option value="-">Subtraction (-)</option>
            <option value="*">Multiplication (*)</option>
            <option value="/">Division (/)</option>
        </select>

        <input type="number" id="num2"
               placeholder="Enter second operand">

        <button onclick="calculate()">Calculate</button>

        <div id="result">Result:</div>

    </div>

    <script>
        function calculate() {

            let num1 = parseFloat(
                document.getElementById("num1").value
            );

            let num2 = parseFloat(
                document.getElementById("num2").value
            );

            let op = document.getElementById("operator").value;
            let result;

            if (isNaN(num1) || isNaN(num2)) {
                document.getElementById("result").innerHTML =
                    "Please enter valid numbers.";
                return;
            }

            switch (op) {

                case "+":
                    result = num1 + num2;
                    break;

                case "-":
                    result = num1 - num2;
                    break;

                case "*":
                    result = num1 * num2;
                    break;

                case "/":
                    if (num2 === 0) {
                        document.getElementById("result").innerHTML =
                            "Cannot divide by zero.";
                        return;
                    } else {
                        result = num1 / num2;
                    }
                    break;

                default:
                    result = "Invalid operator";
            }

            document.getElementById("result").innerHTML =
                "Result: " + result;
        }
    </script>

</body>
</html>