Code

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace SoftUniiii
{
    class Program
    {

        static void swap(ref int num1, ref int num2)
        {
            int temp = num1;
            num1 = num2;
            num2 = temp;
        }
        static int GCD(int a, int b)
        {
            for (int i = Math.Min(a, b); i >= 1; i--)
            {
                if (a % i == 0 && b % i == 0)
                {
                    return i;
                }
            }
            return 1;
        }

        static int Euclid1GCD(int a, int b)
        {
            while (a != b)
            {
                if (a > b)
                {
                    a -= b;
                }
                else
                {
                    b -= a;
                }
            }
            return a;
        }

        static int Euclid2GCD(int a, int b)
        {
           while (a != b)
           {
                if (a < b)
                {
                    swap(ref a, ref b);
                }
                a -= b;
           }
            return a;
        }

        static int Euclid3GCD(int a, int b)
        {
            while (b != 0)
            {
                a %= b;
                swap(ref a, ref b);
            }
            return a;
        }
        static double CircleArea(double radius)
        {
            double area = Math.PI * Math.Pow(radius, 2);
            return area;
        }

        static int LCM(int a, int b)
        {
            for (int i = Math.Max(a, b); i <= a * b; i++)
            {
                if (i % a == 0 && i % b == 0)
                {
                    return i;
                }
            }
            return a * b;
        }

        static void Main(string[] args)
        {
            int a = 20, b = 10, c = 15;
            int g = GCD(GCD(a, b), c);
            Console.WriteLine(g);
        }
    }
}