-- see if the file exists
local function file_exists(file)
	local f = io.open(file, "rb")
	if f then
		f:close()
	end
	return f ~= nil
end

-- get all lines from a file, returns an empty
-- list/table if the file does not exist
local function lines_from(file)
	if not file_exists(file) then
		return {}
	end
	local lines = {}
	for line in io.lines(file) do
		lines[#lines + 1] = line
	end
	return lines
end

-- tests the functions above
local file = "input"
local lines = lines_from(file)

local left_list = {}
local right_list = {}

-- https://www.lua.org/manual/5.4/manual.html#pdf-ipairs
-- print all line numbers and their contents
for _, v in ipairs(lines) do
	for n1, n2 in string.gmatch(v, "(%d+)%s+(%d+)") do
		-- [](https://stackoverflow.com/a/27434198)
		-- [Lua Length Operator](https://www.lua.org/manual/5.2/manual.html#3.4.6)
		left_list[#left_list + 1] = n1
		right_list[#right_list + 1] = n2
	end
end

local right_list_count_map = {}

for _, v in ipairs(right_list) do
	if right_list_count_map[v] ~= nil then
		right_list_count_map[v] = right_list_count_map[v] + 1
	else
		right_list_count_map[v] = 1
	end
end

local total = 0

for _, v in ipairs(left_list) do
	local count
	if right_list_count_map[v] ~= nil then
		count = right_list_count_map[v]
	else
		count = 0
	end
	total = total + (v * count)
end

io.output():write(total)