You can attain this by adding edges. Propose a strategy for solving this with adding as little edges as possible A Vertice on the input graph can be without edges.
Keep in mind that the algorithm doesn't need to be perfect, also NO NEED to code anything (unless you want to:)).
Consider a DAG that has m vertices with in-degree 0 and n vertices with out-degree 0:
Obviously, you will need to add m edges incoming to the in-degree 0 vertices, and n edges outgoing from the out-degree 0 vertices, so the best you can hope for is max(m,n) new edges. We can achieve this.
First, if m > n, then connect in-degree 0 vertices until m = n. Otherwise connect out-degree 0 vertices until m = n.
Now we have m=n, which we will maintain: