void build(int id = 1,int l = 0,int r = n){
	if(r - l < 2){	//	l + 1 == r
		s[id] = a[l];
		return ;
	}
	int mid = (l+r)/2;
	build(id * 2, l, mid);
	build(id * 2 + 1, mid, r);
	s[id] = s[id * 2] + s[id * 2 + 1];
}