import std;

void main()
{
    File("input", "r").byLine.findMissingSeatID.writeln;
}

auto calculateSeatID(const char[] seat)
{
    return seat.retro.enumerate.map!(it => "BR".canFind(it.value) ? 2 ^^ it.index : 0).sum;
}

unittest
{
    assert("FBFBBFFRLR".calculateSeatID == 357);
    assert("BFFFBBFRRR".calculateSeatID == 567);
    assert("FFFBBBFRRR".calculateSeatID == 119);
    assert("BBFFBBFRLL".calculateSeatID == 820);
}

auto findMissingSeatID(T)(T seats)
{
    auto sortedSeatIDs = seats.map!calculateSeatID.array.sort;
    return sortedSeatIDs.zip(sortedSeatIDs.drop(1))
        .find!(seatPair => seatPair[1] - seatPair[0] != 1).front[0] + 1;
}