#include <iostream>
#include <string>
#include <vector>
#include <math.h>
#include <algorithm>
#define rep(i,n) for (int i = 0; i < (n); ++i)
using ll = long long;
using namespace std;
int main()
{
	int r;
	cin >> r;
	cout << 2*r*3.141592 << endl;
	return 0;
}
