Sum

var numbers = new[] {1,2,3,4};

var sumOfAllNumbers = numbers.Sum();
Console.WriteLine(sumOfAllNumbers); //10

var cities = new[] {
    new {Population = 1000},
    new {Population = 2500},
    new {Population = 4000}
};

var totalPopulation = cities.Sum(c => c.Population);
Console.WriteLine(totalPopulation); //7500