using System;

public class Program

{

    public static void Main()

    {

        string parsedDateTimeString;

        if(TryChangeDateTimeFormat("2018-6-18 20:50:35", "yyyy-M-d hh:mm tt", out parsedDateTimeString))

        {

            Console.WriteLine(parsedDateTimeString);

        }

        else

        {

            Console.WriteLine("There was an issue");

        }

    }

    public static bool TryChangeDateTimeFormat(string inputDateString, string outputFormat, out string outputDateString)

    {

        DateTime parsedDateTime;

        if(DateTime.TryParse(inputDateString, out parsedDateTime))

        {

            outputDateString = parsedDateTime.ToString(outputFormat);

            return true;

        }

        outputDateString = string.Empty;

        return false;

    }

}