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module halfadder (a, b, sum, cout);
    input a, b;
    output sum, cout;

    and carry(cout, a, b);
    xor (sum, a, b);

endmodule



module fulladder (a, b, cin, sum, cout);
    input a, b, cin;
    output sum, cout;

    wire carry1, carry2, sum1;

    halfadder h1 (a, b, sum1, carry1);
    halfadder h2 (cin, sum1, sum, carry2);

    or (cout, carry1, carry2);

endmodule

module SubtractorCircuit (a1, b1, a2, b2, a3, b3, a4, b4, cin, coutinv,sum11, sum22, sum33, sum44);

    input a1, b1, a2, b2, a3, b3, a4, b4, cin;
    output sum11, sum22, sum33, sum44,coutinv;

    wire carry1, carry2, carry3;
	 wire b11,b22,b33,b44, cout;
	 wire sum1, sum2, sum3, sum4;

	 not(b11,b1);
	 not(b22,b2);
	 not(b33,b3);
	 not(b44,b4);
	 not(coutinv,cout);
	 not(sum11,sum1);
	  not(sum22,sum2);
	  not(sum33,sum3);
	  not(sum44,sum4);

    fulladder f1(a1, b11, cin, sum1, carry1);
    fulladder f2(a2, b22, carry1, sum2, carry2);
    fulladder f3(a3, b33, carry2, sum3, carry3);
    fulladder f4(a4, b44, carry3, sum4, cout);

endmodule