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