拿到2顆紅球的機率與黑球相等,故500/2001*2=1000/2001拿兩顆相異情形,只須考慮第2顆相異即可,機率是1001/2001 即m=2C(1001,2)/C(2002,2)=1000/2001 N=1-M=1001/2001