#include <bits/stdc++.h>
using namespace std;
using ll = long long;
#define _GLIBCXX_DEBUG
#define rep(i, c) for (int i = 0; i < (int)c; i++)
#define debug(x) cout << #x << " = " << x << endl
#define print(x) cout << x << endl;
const ll MOD = 1000000007;

int main()
{
    int R;
    cin >> R;
    const double PI = 3.14159265358979323846;
    cout << fixed << setprecision(30);
    cout << 2 * R * PI << endl;
}