Let $Z_1, Z_2, \dots, Z_n$ be i.i.d. Bernoulli$(p)$ trials. Define $X = \sum_{i=1}^{k} Z_i$ and $Y = \sum_{i=m}^{n} Z_i$, where $1 < m \le k < n$. What is the covariance between $X$ and $Y$?