#include <bits/stdc++.h>
using namespace std;
#define Hello ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0);
#define ll long long
#define endl '\n'
const double pi = acos(-1);
int main()
{
    Hello
    //2pir
    int n;
    cin >> n;
    cout << fixed << setprecision(2) << 2.0 * pi * n;
    return 0;
}