69. Binary Subtractor

How do you plan to solve it?

/*Write your code here*/
module sub4_2c(
    input [3:0]a,
    input [3:0]b,
    output [3:0]diff,
    output bout
);
wire[4:0] sum5;
assign sum5={1'b0,a}+{1'b0,~b}+ 5'b00001;
assign diff= sum5[3:0];
assign bout=~sum5[4];
endmodule