#define _USE_MATH_DEFINES
#include <math.h>
#include <iostream>
#include <iomanip>
#include <cstdlib>
#include <algorithm>

using namespace std;
typedef long long ll;

#define rep(i, n) for(ll i = 0; i < (ll)(n); i++)

int main() {

    ll R;
    cin >> R;
    cout << 2*R*M_PI << endl;
    return 0;
}
