2^n =2*2^（n-1）
2-2²-2³-2^4-2^5-2^6-2^7-2^8-2^9+2^10
=2-2²-2³-2^4-2^5-2^6-2^7-2^8+2^9
=2-2²-2³-2^4-2^5-2^6-2^7+2^8
……
=2+2²=2+4=6