From one of my infamous quotes (as @Kenshin13 says): Learn programming (C# more specifically) JK
It says that the format you're trying to convert using int.Parse is not a valid format (aka isn't a integer). Either fix the input, or add a try/catch on your code. Inb4, if you ask me what is try/catch, read the first line of this post again.
Also, Google is a wonderful tool you know