Special numbers
Practice
2.8 (33 votes)
Basic programming
Math
Number theory
Problem
89% Success 7588 Attempts 20 Points 1s Time Limit 256MB Memory 1024 KB Max Code
A natural number is special if it consists of only digits \(4\) and \(7\). Find the number of pairs of сoprime special numbers \((x,\ y)\) such that \(1 ⩽ x ⩽ y ⩽ N\).
Input format
The first line contains an integer \(N\) (\(1 ⩽ N ⩽ 10^9\)).
Output format
Print a single number denoting the answer to the problem.
Sample Input
10
Sample Output
1
Explanation
Clarification: https://en.wikipedia.org/wiki/Coprime_integers
Answer = 1 because 4 and 7 \(\le\) 10 and gcd(4,7) = 1
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