#include<iostream>
#include<math.h>
using namespace std;
int main(void)
{long double pi=1,si,i;

for(i=1;i<100000;i++)
    {si = (pow(-1.0 ,i))/(2*i+1);
    pi += si;
    }

pi*=4.0;
cout<<"  "<< pi <<" n ";

}