i hate maths [CTF]

// @Author -> [email protected] [n0va]

function evaluate_expression(){

    //Selecting the Expression
    let selectorBox = $(".message").text();

    // Extracting the Expression 3877 + 0x213d - 1111011011001
    selectorBox = selectorBox.split("\n")[2].substring(2,selectorBox.lenght);

    // getting individual Values to play with
    let arr = selectorBox.split(' ');

    // 0 , 2, 4 are the numbers holder
    let a = arr[0]; let b = arr[2]; let c = arr[4];

    // string to number
    a = parseInt(a);

    b = parseInt(b, 16); // converting hex to number [0x213d -> 8509]

    c = parseInt(c, 2).toString(16); // binary to hex  [1111011011001 -> 1ed9]
    c = parseInt(c, 16); // hex to decimal [1ed9 -> 7897]

    return a + b - c; //work finished
}

window.location.href += '/' + evaluate_expression();