#include <iostream>
#include <iomanip>
#include <unordered_map>
#include <set>
#include <fstream>
#include <cstring>
#include <climits>
#include <algorithm>
#include <math.h>
#include <vector>
using namespace std;

#define ll long long
#define mod 1000000007

int main() {

	ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0);
#ifndef ONLINE_JUDGE
	freopen("input.txt", "r", stdin);
	freopen("output.txt", "w", stdout);
#endif

	int r;
	cin >> r;
	cout << setprecision(14) << 2.0 * 3.141592653589793238 * (double)r;

	return 0;
}