#include <iostream>
#include <algorithm>
#include <utility>
#include <vector>
#include <string>
#include <map>
#include <queue>
#include <cmath>

using namespace std;
typedef long long ll;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
#define REP(i, n) for (int i = 0; i < (n); i++)
#define RREP(i, n) for (int i = (n) - 1; i >= 0; i--)
#define FOR(i, a, b) for (int i = (a); i < (b); i++)
#define RFOR(i, a, b) for (int i = (a); i > (b); i--)
#define ALL(a) (a).begin(), (a).end()

int main() {
    double R;
    cin >> R;
    cout << M_PI * R * 2 << endl;
    return 0;
}