pages.filterNot { it.isValid() }
                .map { line ->
                    fun order(sorted: List<String>, left: List<String>): List<String> {
                        if (left.isEmpty()) return sorted
                        val next = left.find { n -> order.none { (a, b) -> b == n && a in left } }!!
                        return order(sorted + next, left - next)
                    }
                    order(emptyList(), line)
                }.sumOf { it[it.size / 2].toInt() }