To solve this problem, we need to determine the number of distinct subsequences of length 2 from a given string (assuming the string consists of distinct characters).

Approach

A subsequence of length 2 is formed by choosing any two distinct characters from the string, regardless of their positions (as long as their order is preserved, but since we're counting distinct pairs, the order might not matter here if the problem considers unordered pairs, but for ordered pairs, it would be different—however, the answer 3 implies unordered pairs).

For a string with n distinct characters, the number of distinct unordered subsequences of length 2 is given by the combination formula C(n,2) = n*(n-1)/2.

If the input string (e.g., "abc") has 3 distinct characters, then C(3,2) = 3*2/2 = 3, which matches the answer.

Solution Code

# Assuming the input string is "abc" (or any 3 distinct characters)
s = input().strip()
n = len(set(s))  # if all characters are distinct, len(set(s)) = len(s)
result = n * (n-1) // 2
print(result)

Explanation

• Combination Calculation: The formula C(n,2) calculates the number of ways to choose 2 elements from n distinct elements without considering order.
• Example: For the string "abc", the valid 2-length subsequences are "ab", "ac", "bc"—total of 3, which is the result.

Answer: 3

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