#include<bits/stdc++.h>
using namespace std;
int main() {
	double s=0;
	for(int i=1;; i+=2) {
		double t=1.0/i;
		if(fabs(t)<1e-6)
			break;
		if(i%4==1)
			s+=t;
		else
			s-=t;
	}
	cout<<"PI="<<fixed<<setprecision(8)<<4*s<<endl;
	return 0;
}