using System;
using System.Globalization;
using System.Linq;
using System.Collections.Generic;

public class Test
{
    

	public static void Main()
	{
        string myString = "words words words FIRSTPHRASE these words I want SECONDPHRASE but not these words";
        int indexOfFirstPhrase = myString.IndexOf("FIRSTPHRASE");
        string result = myString;
        if(indexOfFirstPhrase >= 0)
        {
            indexOfFirstPhrase += "FIRSTPHRASE".Length;
            result = myString.Substring(indexOfFirstPhrase);
            int indexOfSecondPhrase = myString.IndexOf("SECONDPHRASE", indexOfFirstPhrase);
            if (indexOfSecondPhrase >= 0)
            {
                result = myString.Substring(indexOfFirstPhrase, indexOfSecondPhrase - indexOfFirstPhrase);
            }
        }
        Console.WriteLine(result);
	}
}