#include "bits/stdc++.h"
using namespace std;

#define fi first
#define se second
#define pb push_back
#define int long long
#define all(x) (x).begin(), (x).end()

using LL = long long;
using LD = long double;
using pii = pair<int, int>;
using vii = vector<pii>;

const int INF = 1e18;
const int MOD = 1e9 + 7;
const int N = 1e5 + 5;

int32_t main() {
    //ios::sync_with_stdio(false); cin.tie(nullptr);

    double r;
    cin >> r;

    double pi = acos(-1);
    cout << fixed << setprecision(10);
    cout << 2*pi*r << "\n";
}