Problem:
Euler's Phi Function

For given integer n, count the totatives of n, that is, the positive integers less than or equal to n that are relatively prime to n.

Input

n

An integer n (1 ≤ n ≤ 1000000000).

Output

The number of totatives in a line.

Sample Input 1

6

Sample Output 1

2

Sample Input 2

1000000

Sample Output 2

400000

Solution in C# (judge verdict: Time Limit Exceeded)

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
class Program
{
    public static bool f = false,t=true;
    static void Main(string[] args)
    {int sum=0;
long n=long.Parse(Console.ReadLine());
for(int i=1;i<n;i++){
if(gcd(n,i)==1){sum++;}
}
Console.WriteLine(sum);
    }
 
    public static long gcd(long a, long b)
    {
        while (true)
        {
            if (a == 0) { return b; }
            if (b == 0) { return a; }
            if (a > b) { a -= b; }
            else { b -= a; }
        }
    }
    public static long lcm(long a, long b)
    {
        return a  / gcd(a, b) * b;
    }
    public static bool IsPrime(long n)
    {
        if (n < 2) return false;
        if (n == 2) return true;
 
        for (long i = 2; i < n; i++)
        {
            if (n % i == 0) return false;
        }
        return true;
    }
 
}
public static class Read
{
    public static void outl(string n = "") { Console.WriteLine(n); return; }
    public static long Long()
    {
        return long.Parse(Console.ReadLine());
    }
    public static long[] longs()
    {
        return Console.ReadLine().Split().Select(long.Parse).ToArray();
    }
    public static int Int()
    {
        return int.Parse(Console.ReadLine());
    }
    public static int[] ints()
    {
        return Console.ReadLine().Split().Select(int.Parse).ToArray();
    }
    public static string Str() { return Console.ReadLine(); }
    public static string[] Strs() { return Console.ReadLine().Split(); }
}