[https://telegra.ph/UniLecs-127-Intervalnaya-summa-09-16]
using System; public class Program { public static long SumOfRange(long a, long b) { long result = (a + b) * (b - a + 1) / 2; return result; } public static void Main() { Console.WriteLine("UniLecs"); Console.WriteLine(string.Format("Answer = {0}", SumOfRange(1, 2))); // 3 Console.WriteLine(string.Format("Answer = {0}", SumOfRange(1, 3))); // 6 Console.WriteLine(string.Format("Answer = {0}", SumOfRange(2, 4))); // 9 } }