Overview

This problem asks you to count the number of students among \(N\) students whose scores are below the passing threshold of \(X\) points.

Analysis

This problem is very simple and can be solved by checking each student’s score one by one and determining whether it is below the passing threshold.

Key Observations

• The expression “below” means \(A_i < X\) (note that it is not \(A_i \leq X\))
• For example, if the passing threshold is \(60\) points:
  • A student with \(59\) points → needs remedial lessons (\(59 < 60\))
  • A student with \(60\) points → does not need remedial lessons (\(60 < 60\) is false)
  • A student with \(61\) points → does not need remedial lessons

Why a Naive Approach is Sufficient

In this problem, \(N \leq 2 \times 10^5\), so an \(O(N)\) algorithm that checks each student’s score once is fast enough. No special techniques are needed.

Algorithm

1. Read \(N\) (number of students) and \(X\) (passing threshold)
2. Read each student’s score \(A_1, A_2, \ldots, A_N\)
3. For each score \(A_i\), determine whether \(A_i < X\)
4. Count and output the number of students satisfying the condition

Concrete Example

Sample input:

5 60
45 60 70 55 80

• Student 1: \(45 < 60\) → needs remedial lessons ✓
• Student 2: \(60 < 60\) → false, so no remedial lessons needed
• Student 3: \(70 < 60\) → false, so no remedial lessons needed
• Student 4: \(55 < 60\) → needs remedial lessons ✓
• Student 5: \(80 < 60\) → false, so no remedial lessons needed

Answer: \(2\) students

Complexity

• Time complexity: \(O(N)\)
  • Because we check each student’s score once
• Space complexity: \(O(N)\)
  • Because we store all students’ scores in a list

Implementation Tips

• In Python, you can use the generator expression sum(1 for a in A if a < X) to concisely count the number of elements satisfying the condition

• This is equivalent to the following code:

  
  count = 0
  for a in A:
    if a < X:
        count += 1
  print(count)
  

• Be careful not to confuse the comparison operators < and <= (“below” means <)

  Source Code

N, X = map(int, input().split())
A = list(map(int, input().split()))
print(sum(1 for a in A if a < X))

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