/* Viet chuong trinh nhap vao 2 so x, y va 1 trong 4 toan tu +, -, *, /.
Neu la + thi in ra ket qua x+y, neu la - thi in ra x-y, neu la * thi
in ra x*y, neu la / thi in ra x/y.(neu y=0 thi in thi...