ADD ADS MIDDLE OF CONTENT

Hello I want to give ads Middle of my content but rules should be work like this After 100 characters show my ads! For example AFTER TWO CHARACTERS ADS EXAMPLE IN HERE MY CONTENT IS ABCDFEG it should be AB"ADSHERE"CDFEG ,so as you see after 2 character it showed my ads. 1) Please Dont provide to me any plugin 2) that php script should be working for characters NOT paragraph 3) Do not copy paste anything from google directly firstly you try then mg me.