JSFiddle - React, Tailwind, and code Playground

JavaScript

function findMissingPages(remainingPageSum) {
    var pageNumber = 1;
    var totalPageSum = pageNumber;
    
    while (totalPageSum < remainingPageSum) {
        pageNumber += 1;
        totalPageSum += pageNumber;
    }
    var missingPageSum = totalPageSum - remainingPageSum;
    
    return [
        Math.floor(missingPageSum / 2),
        Math.ceil(missingPageSum / 2)
    ];
}

var missingPages = findMissingPages(81707);

alert("Pages " + missingPages + " are missing.");