I write in code,I create app and other useful thing like calculator
HTML

```html
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>Calculator</title>
<style>
body {
font-family: Arial, sans-serif;
text-align: center;
}
.calculator {
width: 300px;
margin: 0 auto;
border: 2px solid #ccc;
border-radius: 5px;
padding: 20px;
}
input[type="text"] {
width: 100%;
padding: 10px;
margin-bottom: 10px;
border-radius: 5px;
border: 1px solid #ccc;
}
input[type="button"] {
width: 45px;
height: 45px;
margin: 5px;
font-size: 18px;
border-radius: 5px;
border: 1px solid #ccc;
cursor: pointer;
}
</style>
</head>
<body>
<div class="calculator">
<input type="text" id="display" readonly>
<br>
<input type="button" value="7" onclick="appendToDisplay('7'">
<input type="button" value="8" onclick="appendToDisplay('8'">
<input type="button" value="9" onclick="appendToDisplay('9'">
<input type="button" value="/" onclick="appendToDisplay('/'">
<br>
<input type="button" value="4" onclick="appendToDisplay('4'">
<input type="button" value="5" onclick="appendToDisplay('5'">
<input type="button" value="6" onclick="appendToDisplay('6'">
<input type="button" value="*" onclick="appendToDisplay('*'">
<br>
<input type="button" value="1" onclick="appendToDisplay('1'">
<input type="button" value="2" onclick="appendToDisplay('2'">
<input type="button" value="3" onclick="appendToDisplay('3'">
<input type="button" value="-" onclick="appendToDisplay('-'">
<br>
<input type="button" value="0" onclick="appendToDisplay('0'">
<input type="button" value="." onclick="appendToDisplay('.'">
<input type="button" value="=" onclick="calculate()">
<input type="button" value="+" onclick="appendToDisplay('+'">
<br>
<input type="button" value="C" onclick="clearDisplay()">
</div>

<script>
function appendToDisplay(value) {
document.getElementById("display".value += value;
}

function calculate() {
let expression = document.getElementById("display".value;
let result = eval(expression);
document.getElementById("display".value = result;
}

function clearDisplay() {
document.getElementById("display".value = "";
}
</script>

</body>
</html>
```

This HTML code creates a simple calculator with basic arithmetic operations. You can click on the buttons to input numbers and operators, and the result will be displayed on the text input.