<?php
$input = explode(',', file_get_contents('input.txt'));

// don't forget : 12 @ position 1 && 2 @ position 2

$i = 0;
while($input[$i] !== 99) {
    [$operation, $first, $second, $position] = array_slice($input, $i, 4);
    switch ($operation) {
        case 1:
            $input[$position] = $input[$first] + $input[$second];
            break;
        case 2:
            $input[$position] = $input[$first] * $input[$second];
            break;
        case 99:
            echo $input[0];
            exit;
        default:
            echo 'unknown operation : '.$operation;
            exit;
    }
    $i += 4;
}

echo $input[0];