Help!!

I have been looking for a manga for some time. Guy A He has I’m going to call her a nanny that told him to write like a message and tie it on a balloon and someone will find it and that person will be his friend because he was lonely and didn’t have any friends. Guy B finds the letter in a tree and opens it and reads it. They started to send letters to each other and planned on meeting each other. At the time I read it it was still incomplete but I haven’t been able to find it since then.