Graph Algorithms · Challenge problem

A graph has vertices $m,n,o,p,q,r$ and weighted edges $mn{:}4, mo{:}7, no{:}3, op{:}6, oq{:}5, pq{:}2, pr{:}8, qr{:}7$, together with one more edge $np$ whose weight is currently $9$ (so $np$ is not part of the minimum spanning tree). If $np$'s weight were lowered, to what value would it need to drop so that the graph FIRST has two different minimum spanning trees (a tie)?