using System;

using System.Text.RegularExpressions;

public class Program

{

    public static bool IsNumber(string str)

    {

        return new Regex(@"^[0-9]+").IsMatch(str);

    }

    public static void Main()

    {

        Console.WriteLine(IsNumber("123"));

        Console.WriteLine(IsNumber("abc"));

        Console.WriteLine(IsNumber("1abc"));

        Console.WriteLine(IsNumber("a123"));

    }

}