#include<bits/stdc++.h> 
#include<stdio.h>
using namespace std; 

int main() {
ios::sync_with_stdio(false);
  			cout.tie(nullptr);
  			cin.tie(nullptr);


  			int r;
  			cin >> r;
  			double cir = 2 * M_PI * (double) r;
  			cout << fixed << setprecision(10) << cir;
}