#!/usr/bin/env bash

sed 's/^A/B/g;s/A$/B/g' input.txt | paste -s -d '' | 
  perl -lne '
    print "$1$4$2$3" 
    while 
      /(?<=(.).(.).{138})A(?=.{138}(.).(.))/g
  ' | 
  grep -Ec '(SM|MS){2}'