#include<cstdio>
#include<iostream>
#include<algorithm>
#include<cmath>
#include<set>
#include<vector>
using namespace std;
#define PI acos(-1)
int main(){
    double r;
    double cum;
    cin>>r;
    cum=2*PI*r;
    printf("%.20f",cum);
    
    
    
    
    
    
}