#include<iostream>
#include<string>
#include<vector>
#include<algorithm>
using ll = long long;
using namespace std;
#define rep(i,n) for(int i=0;i<n;i++)

int main() {
	float r;
	cin >> r;
	float p;
	p = r *2* 3.14159265;
	cout << p << endl;
	return 0;
}