Merge Replication Conflicts

Question

Example:

In the table [article table] , the column [article stock] is 5
subscriber 1 sells 1 unit and updates [article stock] to 4
subscriber 2 sells 2 units and updates [article stock] to 3

when they synchronize, there will be a conflict on [article stock], and none of them is right.
The right value should be 5-1-2 = 2

How can i accomplish this?

Thanks for your help!