using System;

using System.Linq;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        var list = new List<Obj>();

        list.Add(new Obj(2,false));

        list.Add(new Obj(2,false));

        list.Add(new Obj(2,false));

        list.Add(new Obj(2,false));

        list.Add(new Obj(2,true));

        list.Add(new Obj(2,true));

        var res = list.Sum(o => o.flag == true ? o.val : o.val * -1);

        res.Dump();

    }

    class Obj {

        public int val { get; set; }

        public bool flag { get; set; }

        public Obj(int val, bool flag) {

            this.val = val;

            this.flag = flag;

        }

    }

}