João and Maria each have a large jug with one liter of water. On the first day, João puts $1 \mathrm{ml}$ of water from his jug into Maria's jug. On the second day, Maria puts $2 \mathrm{ml}$ of water from her jug into João's jug. On the third day, João puts $3 \mathrm{ml}$ of water from his jug into Maria's jug, and so on. After 200 days, how many milliliters of water are in Maria's jug?
Initially, the volume of water in Maria's jar is $1 \mathrm{l}=$ $1000 \mathrm{ml}$. After 200 days, the volume is the same, increased by what João added and decreased by what she took to put in João's jar, that is,

$$
\begin{aligned}
1000 & +1-2+3-4+\cdots+199-200 \\
& =1000+(1-2)+(3-4)+\cdots+(199-200) \\
& =1000-(\underbrace{1+\cdots+1}_{100})=900
\end{aligned}
$$

Therefore, after 200 days, Maria will have $900 \mathrm{ml}$ in her jar.