program main
    implicit none

    integer::k,n,f,a(100),x=0 ,i
     read(*,*) k,n,f
     read(*,*) a(1:f)
      do i = 1,f
      x = x + a(i)
     end do
     x = k* n - x
   if(x>= 0) then
   write(*,*) x
   else if(x<0) then
   write(*,*) "-1"
   end if
  stop
end program main