69. Binary Subtractor

How do you plan to solve it?

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