奇数1,3,5,7,9,11,13,15,17,19
偶数2,4,6,8,10,12,14,16,18,20
质数2,3,5,7,11,12,17,19
合数4,6,8,9,10,12,14,15,16,18,20
1即不是质数,也不是合数