#include <bits/stdc++.h>
#include <algorithm>
#define rep(i, n) for (int i=0; i<n; ++i)
#define all(obj) (obj).begin(),(obj).end()
using namespace std;
typedef long long ll;

int main(){
    int R;
    cin >> R;
    cout << R*2*3.141592 << endl;
 }
