--Lua 5.4.2
local File = io.open("input.txt", "r")

if not File then
    print("File not found")
    return
end

local LeftList = {}
local RightList = {}

for Line in File:lines() do
    local LeftId, RightId = Line:match("(%d+)%s+(%d+)")
    table.insert(LeftList, tonumber(LeftId))
    table.insert(RightList, tonumber(RightId))
end

File:close()

if #LeftList ~= #RightList then
    print("Lists are not the same size")
    return
end

--[[
    Format:
    Appearances[Id] = AppearanceCount
--]]
local Appearances = {}

for ListIndex = 1, #RightList do
    local Id = RightList[ListIndex]
    if not Appearances[Id] then
        Appearances[Id] = 0
    end
    Appearances[Id] = Appearances[Id] + 1
end

local SimilarityScore = 0

for ListIndex = 1, #LeftList do
    local Id = LeftList[ListIndex]
    local AppearanceCount = Appearances[Id]
    if AppearanceCount then
        SimilarityScore = SimilarityScore + Id * AppearanceCount
    end
end

print("Similarity score:", SimilarityScore)