# echo -n '1,9,87,3,10,4,20,2,45' | tr ',' '\n' | sort -g | tr '\n' ' ' | awk -f longest-seq.awk
BEGIN {getline;
  i = 2
  j = 1
  while (i < NF) {
    if ($(i) - $(j) != i - j) {
      j++
    }
    i++
  }
}
END { printf "longest: %s\n", i-j }