import std;

void main()
{
    File("input", "r").byLine.findHighestSeatID.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 findHighestSeatID(R)(R seats)
{
    return seats.map!(seat => seat.calculateSeatID).maxElement;
}