int tr[N];
//初始化 add(i, a[i]);
int lowbit(int x)
{
	return x & -x;
}

void add(int x, int c)
{
	for(int i = x; i <= n; i += lowbit(i))
		tr[i] += c;
}

int ask(int x)
{
	int res = 0;
	for(int i = x; i; i -= lowbit(i))
		res += tr[i];
	return res;
}