#include <bits/stdc++.h>
#define F first
#define S second
using namespace std;
typedef long long ll;
double pi = 2 * acos(0.0);
int main()
{
    ios::sync_with_stdio(0); cin.tie(0); cout.tie(0);
    int r;
    cin >> r;
    cout << fixed << setprecision(12) << 2 * pi * r;
    return 0;
}
