module f1(x1, x0, y1, y0);
	input x1, x0, y1, y0;
	output f;
	assign f = (x1&~y1)|(x0&~y0)|(y1&~x1)|(y0&~x0);

endmodule

module f2(x1, x0, y1, y0);
	input x1, x0, y1, y0;
	output f;
	assign f = (x1|x0|y1|~y0)&(x1|x0|~y1|y0)&(x1|x0|~y1|~y0)&(x1|~x0|~y1|y0)&(x1|~x0|~y1|~y0)&(~x1|x0|~y1|~y0);
endmodule