#include
void main(){ double pi=3.00,t=0.00,s=0.00;
int i=1,j=1;
scanf("%f",&t); //輸入精度控制值
do
{
s =4.00/((2*i)*(2*i+1)*(2*i+2));
pi+= j*s;
i ++;
j *=-1; //符號(hào)控制
}while(s >=t);
printf("%f\n",pi);
}