#include<bits/stdc++.h>
#define PB push_back
#define MP make_pair
#define F first
#define S second
#define MOD	1000000007
#define int	long long
#define pii pair<int,int> 
#define vi vector<int>
using namespace std;

int power(int a, int b){ int S = 1; while(b--)S *= a; return S;}

int32_t main(){
	double r;
	cin >> r;
	cout << 2 * acos(-1.0) * r;

	return 0;
}

