终于过了，纪念一下。

#include<bits/stdc++.h>
#define int long long
#define INF 0x3f3f3f
using namespace std;
int n;
int cnt,a[300001],now=2;
bool primed(int x){
	if(x<2)return 0;
	for(int i=2;i*i<=x;i++)
		if(x%i==0)return 0;
	return 1;
}
signed main(){
	cin>>n;
	if(n==1){
		cout<<"0";
		return 0;
	}
	else if(primed(n)){
		cout<<n-1;
		return 0;
	}
	double m=n;
	while(n!=1){
		bool flag=1;
		while(n%now==0){
			n/=now;
			if(flag){
				a[++cnt]=now;
				flag=0;
			}
		}
		now++;
	}
	for(int i=1;i<=cnt;i++)
		m*=(1-1.0/a[i]);
	int k=m;
	cout<<k;
	return 0;
}

神奇的是，以下代码会 TLE：

#include<bits/stdc++.h>
#define int long long
#define INF 0x3f3f3f
using namespace std;
int n;
int cnt,a[300001],now=2;
bool primed(int x){
	if(x<2)return 0;
	for(int i=1/*here*/;i*i<=x;i++)
		if(x%i==0)return 0;
	return 1;
}
signed main(){
	cin>>n;
	if(n==1){
		cout<<"0";
		return 0;
	}
	else if(primed(n)){
		cout<<n-1;
		return 0;
	}
	double m=n;
	while(n!=1){
		bool flag=1;
		while(n%now==0){
			n/=now;
			if(flag){
				a[++cnt]=now;
				flag=0;
			}
		}
		now++;
	}
	for(int i=1;i<=cnt;i++)
		m*=(1-1.0/a[i]);
	int k=m;
	cout<<k;
	return 0;
}

C++ :

#include<cstdio>
#include<iostream>
#include<cmath>
using namespace std;
long long len=0,n;
double ans,a[100000]={0};
bool judge(long long x)
{
	for(int i=2;i<=sqrt(x);i++)
	if(x%i==0) return 0;
	return 1;
}
int main()
{
	scanf("%d",&n);
	if(!n) 
	{
		cout<<0;
		return 0;
	}
	ans=n;
	if(judge(n)) 
	{
		ans-=1;
		cout<<(long long)ans;
		return 0;
	}
	long long j=2;
	while(n!=1)
	{
		if(!(n%j)) 
		{
			n/=j;
			if(a[len]!=j) a[++len]=j;
			if(judge(n)) 
			{
				if(n!=j) a[++len]=n;
				break;
			}
		}
		else j++;
	}
	for(int i=1;i<=len;i++) ans*=(1-1/a[i]);
	cout<<(long long)ans;
	return 0;
}