#include<iostream>
using namespace std;
int main()
{
    double pai=0.0;
    double s=1.0;
    for(int i=2;i<=200;i++)
    {
       int m=-1;
       s+=1.0/(2*i+1)*m;
       m=-m;
    }
    pai=4*s;
    cout<<pai<<endl;
}