Counting Primes

Problem Background

Special for beginners, ^_^

Problem Description

Write a program to count how many prime numbers there are from 2 to n (where n is a positive integer no less than 2).

Input Format

Input n (2 ≤ n ≤ 50000).

Output Format

Output the number of primes satisfying the requirement.

Sample Input

10

Sample Output

4

Constraints

1s, 1024KiB for each test case.