(40+38+36+...+4+2)-(39+37+35+...+3+1)
=40+38+36+...+4+2 - 39-37-35-...-3-1
=40-39+38-37+36-35+……+4-3+2-1
=1+1+1+……+1+1
总共是20个 1
=20