#include <bits/stdc++.h>
#define int long long
#define rep(i, n) for (int i = 0; i < (int)(n); i++)
using namespace std;
constexpr int MOD = 1000000007;
constexpr int INF = numeric_limits<int>::max() / 2;
typedef pair<int,int> P;
using Graph = vector<vector<int>>;


signed main(){
	cin.tie(nullptr);
	ios::sync_with_stdio(false);

	int R;
	cin >> R;
	cout << fixed << setprecision(10);
	cout << 2* R * 3.14159265 << endl;
}