/*This Code Is Nude*/
#include <bits/stdc++.h>
 
#define pb push_back
#define ll long long
#define int long long
#define max(a,b) max((long long) a , (long long) b)
#define min(a,b) min((long long) a , (long long) b)

using namespace std;

const int inf = 1e12 + 10; 
const int maxn = 2e5 + 10;
const int mod = 1e9 + 7;
 
bool mark [maxn];
vector<int>adj[maxn];

int32_t main(){
	ios:: sync_with_stdio(0), cin.tie(0), cout.tie(0);
	int r;
	cin >> r;
	double p = 3.14;
	cout << r * 2 * p << endl;
	return 0;
}
