pastebin

let numbers = [2, 3, 5, 7, 11, 13, 17, 18, 19];

let str = "";

for (let i = 0; i < numbers.length; i++) {

    if (i % 2 === 1)

        continue;

    str += (str === "" ? "" : "-") + numbers[i];

console.log(`str is "${str}"`);