<?php

$rucksacks = explode("\n", file_get_contents('./input.txt'));

$lettersToNumbers = array_merge(range('a', 'z'), range('A', 'Z'));

$groups = array_chunk($rucksacks, 3);

echo array_sum(
    array_map(
        static function($rucksacks) use ($lettersToNumbers) {
            [$f, $s, $t] = array_map('str_split', $rucksacks);
            return array_search(current(array_intersect($f, $s, $t)), $lettersToNumbers, true) + 1;
        },
        $groups
    )
);