#include <bits/stdc++.h>
#include <math.h>
#define rep(i,n) for (int i=0;i < (n); ++i)
#define ALL(x) (x).begin(), (x).end()
using namespace std;
using ll = long long;
 
 
int main() {
    float n;
    cin >> n;
    cout << 2*n*3.1415 << endl
}