#!/usr/bin/awk -f
# HP20140421
# cat xxx | sort | checkinc
# format: ????nnnn*
BEGIN {
  old = "";
  last = -1;
}
  
{
  cur = substr($0,5,4)+0;
  if (last == -1) {
    last = cur-1;
  }
  if (last != cur-1) {
    print old " -> " $0
  }
  old = $0;
  last = cur;
}