Mark cleaning row n room the ith room nonnegative dust level a_i he magical cleaning machine follow threestep operation Select indice ij dust level a_i a_i1 dot a_j1 strictly great 0 set a_i a_i1 Set a_j a_j1 Marks goal a_1 a_2 ldot a_n1 0 nicely sweep nth room determine minimum number operation need reach goal