program T1
	implicit none
	integer::k,n,f,a(100)
	read(*,*)k,n,f
	read(*,*)a(1:f)	
		if(sum(a(1:f))>k*n)then
			write(*,*)-1
		else
			write(*,*)(k*n)-sum(a(1:f))
		end if
end program T1