program main
    implicit none
    integer :: K, N, F, calc
    integer, allocatable :: A(:)

    read *, K, N, F
    allocate(A(F))
    read *, A(:)

    calc = K * N - sum(A)
    if (calc >= 0) then
        print *, calc
    else
        print *, -1
    end if
end program main