#include <bits/stdc++.h>

using namespace std;
using ll = long long;
using ld = long double;

//#define int long long
//signed main(){
int main() {
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);
    double r;
    cin >> r;
    cout << fixed << setprecision(10) << 2*M_PI*r << "\n";

    return 0;
}
#include <iostream>
#include <math.h>

using namespace std;
void Solve(int a)
{
	const double pi = 3.14;
	float ans;
	ans = 2 * pi * a;
	cout << ans;
}
int main()
{
	int x;
	cin >> x;
	Solve(x);
	return 0;
}