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

int debug = 0;
#define PI 3.14159265359

int main()
{
    int r;
    cin >> r;
    printf("%.20f\n",2*r*PI);
    //     cout << 2 * r * PI << endl;
    return 0;
}