N_m$, respectively. Then there exist ample $\Q$-divisors $A_\ell$ on $S_\ell$ and $A_m$ on $S_m$ such that $$N_\ell\sim_\Q\varphi_\ell^*A_\ell\quad\text{and}\quad N_m\sim_\Q\varphi_m^*A_m.$$ If $\xi$ is a curve on $X$ contracted by $\varphi_m$, then 