[рони]

MC-XOBAHCK,

function fun(start, end) {
    return (start + end) * (Math.abs(end - start) + 1) / 2
}

alert(fun(4, 1));
alert(fun(1, 4)); //1 + 2 + 3 + 4  = 10
alert(fun(5, 2));
alert(fun(2, 5)); //2 + 3 + 4 + 5 = 14