: : 1- 2- 3- 4- 5- 6- 7- 8- : : 1- 2- 3- 4- : : . = : 1 2 ǿ : = - = 2-1=1 = + = 1+2=3 : 3 2 = - 3 - 2 = 1 = - = 2 - 1=1