69. Binary Subtractor

How do you plan to solve it?

/*Write your code here*/
module fa(input a,b,cin,output cout,sum);
assign cout=a&b | b&cin | cin&a;
assign sum=a^b^cin;
endmodule
module sub4_2c(input [3:0]a,b,output bout,output [3:0]diff);
wire [3:0]c;
fa f0(a[0],~b[0],1   ,c[0],diff[0]);
fa f1(a[1],~b[1],c[0],c[1],diff[1]);
fa f2(a[2],~b[2],c[1],c[2],diff[2]);
fa f3(a[3],~b[3],c[2],c[3],diff[3]);
assign bout=~c[3];
endmodule