// C# program to print BFS traversal
// from a given source vertex.
// BFS(int s) traverses vertices
// reachable from s.
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
 
// This class represents a directed
// graph using adjacency list
// representation
class Graph{
     
// No. of vertices    
private int _V;
 
//Adjacency Lists
LinkedList<int>[] _adj;
 
public Graph(int V)
{
    _adj = new LinkedList<int>[V];
    for(int i = 0; i < _adj.Length; i++)
    {
        _adj[i] = new LinkedList<int>();
    }
    _V = V;
}
 
// Function to add an edge into the graph
public void AddEdge(int v, int w)
{        
    _adj[v].AddLast(w);
 
}
 
// Prints BFS traversal from a given source s
public void BFS(int s)
{
     
    // Mark all the vertices as not
    // visited(By default set as false)
    bool[] visited = new bool[_V];
    for(int i = 0; i < _V; i++)
        visited[i] = false;
     
    // Create a queue for BFS
    LinkedList<int> queue = new LinkedList<int>();
     
    // Mark the current node as
    // visited and enqueue it
    visited[s] = true;
    queue.AddLast(s);        
 
    while(queue.Any())
    {
         
        // Dequeue a vertex from queue
        // and print it
        s = queue.First();
        Console.Write(s + " " );
        queue.RemoveFirst();
         
        // Get all adjacent vertices of the
        // dequeued vertex s. If a adjacent
        // has not been visited, then mark it
        // visited and enqueue it
        LinkedList<int> list = _adj[s];
 
        foreach (var val in list)            
        {
            if (!visited[val])
            {
                visited[val] = true;
                queue.AddLast(val);
            }
        }
    }
}

public class King {
    fewf
}

// C# program to implement
// Luhn algorithm
using System;

class GFG {
	
// Returns true if given 
// card number is valid
static bool checkLuhn(String cardNo)
{
	int nDigits = cardNo.Length;

	int nSum = 0;
	bool isSecond = false;
	for (int i = nDigits - 1; i >= 0; i--) 
	{

		int d = cardNo[i] - '0';

		if (isSecond == true)
			d = d * 2;

		// We add two digits to handle
		// cases that make two digits 
		// after doubling
		nSum += d / 10;
		nSum += d % 10;

		isSecond = !isSecond;
	}
	return (nSum % 10 == 0);
}

	// Driver code
	static public void Main()
	{
		String cardNo = "79927398713";
		if (checkLuhn(cardNo))
			Console.WriteLine("This is a valid card");
		else
			Console.WriteLine("This is not a valid card");
	
	}
}