For too long there has been one convenience missing from the city that seems to have every comfort: public toilets. But now the first of 20 self-washing, wheelchair-accessible toilets is expected to open this week in Madison Square Park. It will cost customers 25 cents for 15 minutes of enjoyment before the toilet doors automatically open and a 60-second automatic cleaning cycle begins after each use. The landmark toilet is the first of 20 to be rolled out by Spanish advertising giant Cemusa Inc., which won a bid in 2005 to install the toilets as well as 3,300 new bus shelters and 330 newsstands over a five-year period. Each borough will have one toilet, though the company has not decided on the locations for all of them, Cemusa spokeswoman Katherine Schwab told The New York Sun for its Tuesday edition.