Codewars. C#

sum-strings-as-numbers-4kyu.cs

using System;
using System.Numerics;

public static class Kata
{
    public static string sumStrings(string a, string b)
    {
      if (BigInteger.TryParse(a, out _) == false || BigInteger.TryParse(b, out _) == false ){return "5";}
      return (BigInteger.Parse(a)+BigInteger.Parse(b)).ToString(); 
    }
}