4x+1        (x=4)

         y  =      x+1        (x=1)

                            x       (x=0)

                         -1          other  

        程序：          main(  )

                         {   int    x,      y ;

                               scanf(  "%d",&x );

                           switch(x)

                         {   case     5:      y= 5*x+1  ;  break; 

                             case     4:      y= 4*x+1  ;  break; 

                             case     1:      y= x+1  ;  break; 

                             case     0:      y= x   ;  break;      

                             default   :   y=  -1;

                                    }    

                               printf("y=%d\n",y);

                           }