function multiplyBy2(input) {
    let num = 0;

    for (i = 0; i < input.length; i++) {
        num = Number(input[i]);
        num *= 2;
        if (Math.sign(num) === 1) {
        console.log("Result: " + num.toFixed(2));
        i++;
        } else {
            console.log("Negative number!");
        }
    }
}