<head>chess</head>
<body>
<script>
let lineSize = +prompt("Enter size of table line ");
let str = "";
for (let i = 0; i < lineSize; i++) {
for (let j = 0; j < lineSize; j++) {
str += ((j+i)% 2) == 0 ? "." : "#";
}
str += '\n';
}
alert(str);
</script>
</body>