this is the question i am trying to solve .
this is its editorial .

I didn't quite understood soln in the editorial . I got that they are pre computing knapsack dp of n*n*k
that is knapsack dp of sum . But i didn't understand the fulcrum concept . they say that you have to take
subset from { 1 , 2 , .... ,m-1 } and { 1 , 2, ...... , n-m} . why i mean how will it solve the question ??
what are they trying to say?

Even the smallest help is welcome , i tried asking my friends but they also don't understand the editorial.
thanks in advance.