APN

I had a quick guess, is No.1.
  }$ P5 `( z7 R/ G; J% uBecause what ever he or she taken, the next person will take what ever had taken with +1 or -1 to ensure they havn't got the same number. at the end, no1 will had the most likely middle number and without a same number. of coz he will need to pick a number from 3 to 19.