module adder(A,B,Cin,S,Cout);
input A,B;
input Cin;

wire A,B,Cin;
wire S,Cout;

output S;
output Cout;

assign S = (A ^ B) ^ Cin;
assign Cout = Cin & (A ^ B) | (A & B);

endmodule

module ripple_adder(a,b,cin,sum,cout);
input [3:0] a,b;
input cin;
output [3:0] sum;
output cout;

wire [3:0] a,b;
wire cin;
wire [3:0] sum;
wire cout;
wire [3:0] c;

assign cout = c[3];

adder a0(.A(a[0]), .B(b[0]), .Cin(cin), .S(sum[0]), .Cout(c[0]));
adder a1(.A(a[1]), .B(b[1]), .Cin(c[0]), .S(sum[1]), .Cout(c[1]));
adder a2(.A(a[2]), .B(b[2]), .Cin(c[1]), .S(sum[2]), .Cout(c[2]));
adder a3(.A(a[3]), .B(b[3]), .Cin(c[2]), .S(sum[3]), .Cout(c[3]));

endmodule