<?php
$input    = file('input', FILE_IGNORE_NEW_LINES | FILE_SKIP_EMPTY_LINES);
$template = array_shift($input);
$pairs    = [];
foreach ($input as $item) {
    $line = explode(' -> ', $item);
    $pairs[$line[0]] = $line[1];
}


$newTemplate = $template;
for ($j = 0; $j < 10; $j++) {
    $tmpTemplate = $newTemplate;
    for ($i = 0; $i < strlen($tmpTemplate) - 1; $i++) {
        $window      = substr($tmpTemplate, $i, 2);
        $newTemplate = substr_replace($newTemplate, $pairs[$window], $i + 1 + $i, 0);
    }
}
$characters = array_count_values(str_split(($newTemplate)));
sort($characters);
echo array_pop($characters) - array_shift($characters); //2549