69. Binary Subtractor

How do you plan to solve it?

/*Write your code here*/
module sub4_2c (input wire [3:0] a, b,
                output wire [3:0] diff,
                output wire bout       );
    wire [4:0] tmp;
    assign tmp = a + (~b + 1);
    assign bout = tmp[4];
    assign diff = tmp[3:0];
endmodule