#include <iostream>
#include <math.h>
#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
#define rep(i, n) for (int i = 0; i < (int)(n); ++i)

int main(void)
{
    double r;
    cin >> r;

    cout << 2*M_PI*r << endl;

    return 0;
}
