using static System.Console;

​

public class Program {

    public static void Main() {

        WriteLine(eNumero(""));

        WriteLine(eNumero("a"));

        WriteLine(eNumero("5"));

    }

    public static bool eNumero(string s) {

        if (string.IsNullOrWhiteSpace(s)) return false;

        return (int)s[0] >= (int)'0' && (int)s[0] <= (int)'9';

    }

}