private static string SelectNumberFromString(string input)
{
    string result = "";
    foreach (Match match in Regex.Matches(input, @"\d+"))//不要在匹配字符串的开头和结尾加上"^"和"$"。
    {
        result = match.Value.Length > result.Length ? match.Value : result;
    }
    return result;
}

static void Main(string[] args)
{
    string input = Console.ReadLine();
    string result = SelectNumberFromString(input);
    Console.WriteLine(result);
    Console.ReadKey();
}