Mr. A forgot to write down a very important phone number. All he remembers is that it started with 713 and that the next set of 4 digits involved are 1, 7 and 9 with one of these number and appering twice. He guesses a phone number and dials randomly. The odds against of dialing the correct number is $\frac{m}{n}$. Then least value of  m+n is _____