I'm looking for a proof of Mankiewicz theorem, which states that:
If $U, V$ are open, connected subsets of normed spaces $E, F$ respectively, then every bijective isometry $U \rightarrow V$ extends uniquely to a bijective isometry $E \rightarrow F$.
I know it's available in Bull. Acad. Polon. Sci., 20 (1972), 367–371, but it is not available online.
Could you help me?