#include<iostream>
#include<vector>
using namespace std;
#define rep(i,n) for(int i=0;i<(n);++i)
using ll = long long;
using P = pair<int, int>;
#include<algorithm>
#include<math.h>
#include<map>
#include<queue>
#include<string>
#include<iomanip>

int main() {
	double r;
	cin >> r;
	double M_PI = 3.141592653589793;
	double ans = 2 * r * M_PI;
	cout << fixed << setprecision(8) << ans;

	return 0;
}