solve :: Int -> [Int] -> Int
solve n = uncurry go . (M.fromList &&& last) . flip zip [1..]
   where
   go m (x,i)
      | i == n = x
      | otherwise = go m' (x',i')
      where
      m' = M.insert x i m
      x' = maybe 0 (i -) $ M.lookup x m
      i' = succ i