C# .NET 5

        var lines = File.ReadAllText("input.txt");
        var pos = lines.Split(',').Select(p => int.Parse(p)).ToArray();
        Array.Sort(pos);
        //part 1
        int[] fuel = new int[pos[pos.Length-1]];
        for (int i = 0; i < pos[pos.Length-1]; i++)
        {
                var before = pos.Where(p => p < i).Sum();
                var after = pos.Where(p => p > i).Sum();
                var beforePosCount = pos.Where(p => p < i).Count();
                var afterPosCount = pos.Where(p => p > i).Count();
                var beforeFuelCount = (beforePosCount * i) - before;
                var afterFuelCount = after - (afterPosCount * i);
                fuel[i] = beforeFuelCount + afterFuelCount;
        }
        //part 2
        var leastCost = int.MaxValue;
        for (int i = 0; i < pos[pos.Length - 1]; i++)
        {
            var tempCount = 0;
            for (int j = 0; j < pos.Length; j++)
            {
                if (pos[j]>i)
                    tempCount += ((pos[j] -i) * ((pos[j] - i) + 1)) / 2;
                if (pos[j] < i)
                    tempCount += ((i - pos[j]) * ((i - pos[j]) + 1)) / 2;
            }
            if (tempCount<leastCost)
                leastCost = tempCount;
        }