Codeforces 478D Red Green Towers - The Setup

You're given $r$ red blocks and $g$ green blocks where $1 \le r,$g$\le 2 \cdot 10^5$. A tower of height $h$ uses exactly $1 + 2 + 3 + \dots + h = \frac{h(h+1)}{2}$ blocks total.

You want the tallest tower possible. Find the largest $h$ where $\frac{h(h+1)}{2} \le r + g$. Let $S = \frac{h(h+1)}{2}$. The remaining $r + g - S$ blocks go unused. You don't need to use all blocks. Understanding the problem structure is half the battle.