if(NOT EXISTS "${Data2}")
  message(SEND_ERROR "Input file:\n  ${Data2}\ndoes not exist!")
endif()
foreach(n 1 2 3)
  string(REGEX REPLACE "_1_\\.my\\.dat$" "_${n}_.my.dat" SeriesCFile "${SeriesC}")
  if(NOT EXISTS "${SeriesCFile}")
    message(SEND_ERROR "Input file:\n  ${SeriesCFile}\ndoes not exist!")
  endif()
endforeach()