#include <bits/stdc++.h>

#define nl '\n'
#define F fisrt
#define S second
#define pb push_back
#define mp make_pair
#define REP(i,a,b) for(int i = a; i < b; i++)
#define ForeachTC int t; cin >> t; while(t--)
using namespace std;

typedef long long ll;
typedef vector<int> vi;
typedef vector<ll> vll;
typedef stack<int> si;
typedef deque<int> di;
typedef pair<int,int> pi;

class Solution {
public:
    int climbStairs(int n) {
		if(n <= 2)
			return n;
		else{
			return climbStairs(n-1) + climbStairs(n-2);
		}
    }
};

int main()
{
	ios_base::sync_with_stdio(false);
	cin.tie(NULL);
	
	//code here
	float r; cin >> r;
	cout << 2*M_PI*r;
}
