ABC467 Participation Report

2026/07/18

It is me.

I participated in ABC467. I solved 4 problems. I beat Iwai-seijin. I think this is the second time.

Since both E and F are far beyond the difficulty level I can solve, this looks like quite a good result. I am happy.

Problem A Obesity

The units are confusing, and it is a very untidy problem.

The answer is $W / (\frac{H}{100})^2$ .

Problem B Keep the Change

The answer is the total sum of $B_i - A_i$ when $S_i =$ keep.

Problem C Adjacent Sums (easy)

It strongly smells like DP, but it is not.

Since $A[i+1]$ is determined once $A[i]$ is fixed, you can:

Run a simulation for $A[0] = 0$
Run a simulation for $A[0] = 1$

The answer is the min of these two cases.

Problem D Concentric Circles

Suddenly, geometry. I wish they would stop because it is bad for my heart.

The important things are:

If the dot product of two vectors is $0$ , they are perpendicular.
If the cross product of two vectors is $0$ , they are parallel.

First, if P == R && Q == S, it is Yes because you can use the same circle.

After that, consider whether the perpendicular bisector of $PQ$ and the perpendicular bisector of $RS$ intersect.

If they intersect, you can use that intersection as the common center.

However, calculating the perpendicular bisectors properly is a hassle, so first check if $PQ$ and $RS$ are parallel.

If $PQ$ and $RS$ are not parallel, then their perpendicular bisectors are also not parallel and will always intersect. Therefore, the answer is Yes.

So, does it mean No if $PQ$ and $RS$ are parallel? Not necessarily, because there is a case where the perpendicular bisectors coincide.

You can determine this by checking if the vector connecting the midpoints of $PQ$ and $RS$ is perpendicular to $PQ$ .

If it is perpendicular, the perpendicular bisectors coincide, so the answer is Yes; otherwise, it is No.

That is all.

Final Thoughts

I might have solved Problem E if I could have performed a binary search on ModInt.

For Problem F, the problem to be solved for each query looks like interval scheduling, but maybe I am wrong.

The rate of improvement recently has been abnormal, but I am anxious because my practice cannot keep up at all. It is scary.

See you.