#include <iostream>
#include <bits/stdc++.h>
#define pb push_back
#define mp make_pair
using namespace std;
typedef long long ll;
const int MODU = 1e9+7;
const int N = 1e5+10;

void solve() {
		
}

int main() {
	const double PI = 2*3.1415926;
	cout << setprecision(12);
	int r;
	cin >> r;
	cout << PI * r << endl; 
	return 0;
}