Как получить общую продолжительность в miutes из коллекции списков в C#

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  [TestMethod]
        public void GetTotalDurationFor_PassEventInBetween()
        {
            new List<EventItemModel>()
            {
                new EventItemModel() {DateTime = new DateTime(2017,1,1,10,0,0), Event = EventType.Enter},
                new EventItemModel() {DateTime = new DateTime(2017,1,1,10,30,0), Event = EventType.Pass},
                new EventItemModel() {DateTime = new DateTime(2017,1,1,11,30,0), Event = EventType.Leave},
            }.GetTotalDurationFor().Should().Be(90);
        }
            }

THIS TEST FAILES ...

PLEASE HELP

Что я уже пробовал:

public static double GetTotalDurationFor(this IEnumerable<EventItemModel> lst)
        {
            var selectedEmployeDayinout = lst.OrderBy(d => d.DateTime).ToList();
            const int enterExitOperations = 2;
            double duration = 0;

            if ((selectedEmployeDayinout.Count()%enterExitOperations) != 0) return duration;
            for (var i = 0; i < selectedEmployeDayinout.Count(); i ++)
            {
                var enterDate = selectedEmployeDayinout[i].DateTime;
                var leaveDate = selectedEmployeDayinout[i + 1].DateTime;
                duration += leaveDate.Subtract(enterDate).TotalSeconds;
            }
            return duration;
        }

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