69. Binary Subtractor

How do you plan to solve it?

module sub4_2c(
    input [3:0] a,
    input [3:0] b,
    output  [3:0] diff,
    output  bout
);

wire[4:0] sum5 = {1'b0,a} + {1'b0, ~b} + 5'b00001;
assign diff = sum5[3:0];
assign bout = ~sum5[4];
endmodule