Problem link: Light OJ 1392.

Please give it a read. If I understood correctly, given R, T, N, it asks to find number of pairs (r, t) with 0 ≤ r < R, 0 ≤ t < T such that there exists integers a₁, ..., a_r whose sum is t and each of the numbers a_i is between 1 and N (inclusive). Constraints are 1 ≤ R ≤ 10⁸, 1 ≤ T ≤ 4 * 10⁸, 2 ≤ N ≤ 20.

But then for a fixed r, all t between r and Nr should be reachable (others are not reachable because each of the r variables is at least 1 and at most N). So I ran a brute force (to check), but they don't match the sample.

Code: here.

Interesting enough, the output of the brute force for each sample is exactly 26 more than actual answer. Where did I go wrong?