#include<iostream>
#include<cmath>

#define rep(i, n) for (int i = 0; i < (n); ++i)
#define rep2(i, x, n) for (int i = x; i < (n); ++i)

using namespace std;

int main() {
    const double PI = 4 * atan(1);
    int R;
    cin >> R;

    cout << 2 * PI * R << endl;
    return 0;
}
