Something

on *:text:!calc*:#: { msg $chan $calc($2-)  }
on *:text:!calc*:#: {  //trigger
  if ($2 == $null) {  //if there's no second word
    notice $nick Wtf I've to calculate now?
    halt //halts the script
  } //closes the if statement
  else { //opens an else statement
    msg $chan $calc($2-) 
  } //closes the else
} //closes the script