#include<iostream>
using namespace std;
int main(){
    cout<<"3.1415905109";
}

#include<cmath>
#include<iomanip>
using namespace std;
double  arctanx(double x)
{
	double temp = x, pi=0;  //temp当前数pi 求和
	int i=1,op = 1;       //op用来改变正负号 
	while(temp>=1.0e-6)
	{
		pi+=temp*op;
		op*=-1;
		i+=2;
		temp = pow(x,i)/i;
	 } 
	 return pi;
}
int main()
{
	double pi = 6*arctanx(1/sqrt(3));
	printf("%.10lf",pi);
	//cout<<fixed<<setprecision(10);
	//cout<<pi<<endl;
	return  0;
}

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