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

typedef long long int ll;
typedef vector<int> vi;
typedef set<int> si;
typedef vector<ll> vll;

#define pvec(v) for(auto e: v) cout << e << " ";
#define IO ios_base::sync_with_stdio(false);cin.tie(0);cout.tie(0);
#define forn(s, e) for(int i=s; i<e; ++i)
#define pb push_back
#define INF 1000000000

int main(){
	IO
	long double r, cir;, PI = 3.14159265;
	cin >> r;
	cir = 2.0*PI*r;
	cout << setprecision(6) << fixed << cir << endl;
	return 0;
}
