#include <iostream>
#include <cstdio>
#include <vector>
#include <cmath>

using namespace std;
using ll = long long;
using P = pair<int,int>;
#define rep(i,n) for (ll i = 0; i < (n); ++i)

int main() {
    double n;
    cin >> n;
    cout << fixed << n*(6.28318530717958623200) << endl;
    return 0;
}