//code
module rcadder(
    input logic a1, b1, a2, b2, a3, b3, a4, b4,
    output logic s1, s2, s3, s4, c4
    );
    logic c1, c2, c3;
    fa fa1(a1, b1, 0, s1, c1);
    fa fa2(a2, b2, c1, s2, c2);
    fa fa3(a3, b3, c2, s3, c3);
    fa fa4(a4, b4, c3, s4, c4);
endmodule

module fa(
    input logic a, b, cin,
    output logic sum, cout
    );
    assign sum = a^b^cin;
    assign cout = a*b + b*cin + cin*a;
endmodule

//testbench

module rcadder_tb();
logic a1, b1, a2, b2, a3, b3, a4, b4;
logic s1, s2, s3, s4, c4;

rcadder uut(a1, b1, a2, b2, a3, b3, a4, b4,s1, s2, s3, s4, c4);

initial begin
a1 = 1; a2 = 0; a3 = 1; a4 = 1;
b1 = 1; b2 = 1; b3 = 1; b4 = 1;
#10
a1 = 1; a2 = 0; a3 = 0; a4 = 0;
b1 = 1; b2 = 1; b3 = 1; b4 = 1;
#10
$finish;
end
endmodule