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Special numbers

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2.8 (33 votes)

Basic programming

Math

Number theory

Problem

89% Success 7558 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.

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