#include<bits/stdc++.h>
using namespace std;
#define rep(i,n) for (int i = 0; i < (n); ++i)
#define drep(i,n) for(int i = (n-1); i >= 0; i--)
#define all(v) (v).begin(),(v).end()
template <class T> bool chmax(T &a, const T &b) { if (a < b) { a = b; return 1; } return 0; }
template <class T> bool chmin(T &a, const T &b) { if (b < a) { a = b; return 1; } return 0; }
template <class T> T gcd(T a, T b) { return b ? gcd(b, a % b) : a; }
template <class T> T lcm(T a, T b) { return a/gcd(a,b)*b; }
typedef pair<int, int> P;
typedef long long ll;
const int INF = 1001001001;
const ll LINF = 1001002003004005006ll;
const ll MOD = 1e9+7;

int main() {
    cin.tie(nullptr);
    ios::sync_with_stdio(false);
	int r;
	cin >> r;
    float ans = M_PI * 2 * r;
	cout << ans << "\n";
	return 0;
}
