간단하게 말하면 서로소의 갯수를 구하는 함수

import java.util.*;
import java.io.*;

class Main{
    static long N;

    public static void main(String[] args) throws Exception{
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        N = Long.parseLong(br.readLine());
        long res = N;
        for(long k=2;k<=Math.sqrt(N);k++){
            if(N%k == 0){
                res = res - res/k;
                while(N%k == 0) N /= k;
            }
        }

        if(N>1) res = res - res/N;
        System.out.println(res);
    }
}