#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#include<iostream>
using namespace std;
int main(){
	int x;
	cin>>x;
    float s=1;
    float pi=0;
    float i=1.0;
    float n=1.0;
    while(fabs(i)>=1e-6){
        pi+=i;
        n=n+2;
        s=-s; 
        i=s/n;
    }
    pi=4*pi*x;
    printf("%.10f\n",pi);
    return 0;
}