Codeforces 478D Red Green Towers - Final Answer

After processing all $h$ levels, $dp[h][j]$ tells you: "how many ways to build the full tower using exactly $j$ red blocks." But not every $j$ is valid.

You need: $j \le r$ and $S - j \le g$. The final answer is the sum of all valid $dp[h][j]$:

$\text{answer} = \sum_{\substack{0 \le j \le r \\ S - j \le g}} dp[h][j] \pmod{10^9 + 7}$

Iterating $j$ from $0$ to $S$ and checking both constraints takes $O(S)$ time after the DP is filled.