lab2/ConsoleApp1/ConsoleApp2/Program.cs

int N = Convert.ToInt32(Console.ReadLine());
int a = 0;
if (N > 1)
{
    a = N * (N + 1) / 2;
}
else
{
    for (int i = 1; i <= N; ++i)
    {
        a += i;
    }
}
Console.WriteLine(a);