Link State Routing uses Flooding.
What Flooding means is forwarding a packet that you receive from one interface to all other interface except the one from which the packet was received.
So assuming a node has N edges and it will receive one packet from each edge once, and will forward it to remaining N-1 edges, making a total of N*(N-1) exchanges.
This is the count for only one node, but this happens at all other nodes also which adds a factor of M to the above exchanges, making the total count = M*N*(N-1).
So the total count always has the factor of M*N or in other words it can be expressed in terms of M*N.