[Nexus]

function getArithmeticProgressionSum(max) {
    const min = 1, step = 1;
    if (max < min || max % step > 0) {
        throw new Error('Invalid first argument');
    }
    
    return (min + max) / 2 * (max / step);
}

alert(getArithmeticProgressionSum(2) === 3);
alert(getArithmeticProgressionSum(8) === 36);