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We consider the network communication scenario, over directed acyclic networks with unit capacity edges in which a number of sources si each holding independent unit-entropy information Xi wish to communicate the sum ΣXi to a set of terminals tj. We show that in the case in which there are only two sources or only two terminals, communication is possible if and only if each source terminal pair si/tj is connected by at least a single path. For the more general communication problem in which there are three sources and three terminals, we prove that a single path connecting the source terminal pairs does not suffice to communicate ΣXi. We then present an efficient encoding scheme which enables the communication of ΣXi for the three sources, three terminals case, given that each source terminal pair is connected by two edge disjoint paths.