2D Fenwick Tree — Sum Over Rectangles

void update(int x, int y, int delta) {
  for (int i = x; i <= n; i += i & -i)
    for (int j = y; j <= m; j += j & -j)
      bit[i][j] += delta;
}

long query(int x, int y) {
  long sum = 0;
  for (int i = x; i > 0; i -= i & -i)
    for (int j = y; j > 0; j -= j & -j)
      sum += bit[i][j];
  return sum;
}