05.sh

#! /usr/bin/env bash
A=($(tr 'FBLR' '0101' < "${1:-5.txt}" | sort -nr))
echo "5A: $((2#$A))"
j=; for i in "${A[@]}"; do [[ "${j: -1}" == "${i: -1}" ]] && echo "5B: $((2#$j-1))=$((2#$i+1))"; j=$i; done