#include <iostream>
#include <vector>
#include <string>
#include <algorithm>
#include <cmath>

#include <set>
#include <map>
#include <iomanip>

using namespace std;

#define rep(i, n) for(long long i = 0; i < (long long)(n); i++)
#define all(x) (x).begin(),(x).end()

using ll = long long;

int main()
{
	int r;
	cin >> r;

	cout << r * 2 * 3.141592 << endl;

	return 0;
}