#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 main()
{
    int r;
    cin >> r;
    double ans = 2 * r * M_PI;
    cout << ans << endl;
    return 0;
}
