#include <iostream>
//#include <bits/stdc++.h>
// a&1 check odd or not and a>>=1 halfs a
// for(int& i : vector) cin>>i input
#include <cstdlib>
#include <cstdio>
#include <vector>
#include <queue>
#include <algorithm>
#define loop(i,a,b) for(int i=a;i<b;i++)
#define pb(x) push_back(x)
#define F first
#define S second
using namespace std;

void solve(){
	double pi=3.14159;
	double rad;
	cin>>rad;
	cout<<2*rad*pi<<endl;


}

int main(){
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);
	int t=1;
	//~ cin>>t;
	while(t--){
		solve();
	}
	return 0;
}

