69. Binary Subtractor

How do you plan to solve it?

module sub4_2c(
    input [3:0]a,b,
    output [3:0]diff,
    output bout
);
    wire [4:0] sum = {1'b0, a} + {1'b0, ~b} + 5'b00001;
    assign diff = sum[3:0];
    assign bout = ~sum[4];
endmodule