Riemann Upper and Lower sums of Heaviside function

Question

I've hit a wall with heaviside functions, the question is to find the Riemann sums of, $$\int_{1}^{2}H(x-2)dx$$ and I've worked my way for the partition, $\Delta x = \frac{1}{n}$, hence $M_{i}=1+\frac{1}{n}$.
Then, $$U_n=\Delta x \sum_{i=1}^{n} Mi$$ $$U_n=\frac{1}{n} \times \sum_{i=1}^{n} H(-1+\frac{i}{n})$$ How do i go about evaluating this sum?