BubbleSecretary

The big boss has a new secretary who is rather clumsy and computer illiterate. Once, the boss asked her secretary to make n copies of some text, which contained a very important alphanumeric code of length m. The poor secretary has retyped the whole text n times. A bit later, in each copy she has found one mistyped character (not necessarily a different one). Unfortunately, each time it was one of the characters in the code. To make things worse, the secretary has lost the original text, so the correction cannot be done easily. Please help the secretary to keep her job and reconstruct the correct code if reconstruction is possible.