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

#define db(x) cerr << #x << " == " << x << endl
#define dbs(x) cerr << x << endl
#define _ << ", " <<
#define pb push_back
#define ar array

typedef long long ll;
typedef long double ld;
//using ll = long long;

typedef vector<int> vi;

const ld EPS = 1e-9, PI = acos(-1.);
const int INF = 0x3f3f3f3f, MOD = 1e9+7;

// global to problem

int main() {
	ios::sync_with_stdio(0);
	cin.tie(0);
	cout.tie(0);
	
	int r;
	cin >> r;
	cout << 2*PI*r << endl;

}
