c# LastIndexOf with few words

Question

I'm trying to get LAST index of "hello world" in text string.

            string sourceText = "hello world. no yes what where hello world but not right in left hello world end of text";
            string searchIndex = "hello world";
            int index = sourceText.LastIndexOf(searchIndex);
            if (index != -1)
            {
                Console.Write(index);
            }

but it's always returns -1.