module substractor4 #(parameter BITWIDTH = 4) (A, B, Ci, S, Co);

    // Input and Output Declarations
    input [BITWIDTH-1:0] A;       // A is an input of size BITWIDTH
    input [BITWIDTH-1:0] B;       // B is an input of size BITWIDTH
    input Ci;                     // Carry-in (used for borrow)
    output [BITWIDTH-1:0] S;      // S is an output of size BITWIDTH
    output Co;                    // Carry-out (borrow-out)

    // Internal Wire Declaration
    wire [BITWIDTH:0] Diff;       // Difference wire (one extra bit for borrow)

    // Assign Statements for Subtraction
    assign Diff = A - B - Ci;     // Calculate the difference
    assign S = Diff[BITWIDTH-1:0]; // Assign the lower BITWIDTH bits to S
    assign Co = Diff[BITWIDTH];    // Assign the borrow-out

endmodule