missing.js

function finda(arr) {
    let sumOfNumbers = (arr.length + 1) * (arr.length + 2) / 2;
    let sumOfArr = arr.reduce((a, b) => a + b, 0);
    console.log(sumOfNumbers - sumOfArr)
}

finda([10, 5, 3, 8, 2, 1, 7, 9, 6]);