  Multi-server single-message private information retrieval is studied in the
presence of side information. In this problem, $K$ independent messages are
replicatively stored at $N$ non-colluding servers. The user wants to privately
download one message from the servers without revealing the index of the
message to any of the servers, leveraging its $M$ side information messages. We
assume that the servers only know the number of the side information messages
available at the user but not their indices. We prove a converse bound on the
maximum download rates, which coincides with the known achievability scheme
proposed by Kadhe {\it et. al.}. Hence, we characterize the capacity for this
problem, which is $(1+\frac{1}{N}+\frac{1}{N^2}+\dots+\frac{1}{N^{\left\lceil
\frac{K}{M+1}\right\rceil-1}})^{-1}$. The proof leverages a novel concept that
we call {\it virtual side information}, which, for a fixed query and any
message, identifies the side information that would be needed in order to
recover that message.
