#include<stdio.h>
int main()
{
     double sum=0,i=0,k,j=1;
     while(k<=1e6){
     k=2*i+1;
     sum+=1/k*j;
     j*=-1;
     i+=1;
     }
     sum*=4;
     printf("%.6lf",sum);
     return 0;
}