#include <bits/stdc++.h>
using namespace std;
 
typedef long long ll;
typedef vector<int> vi;
typedef vector<vi> vvi;
typedef vector<ll> vll;
typedef vector<vll> vvll;
typedef vector<string> vs;
typedef vector<bool> vb;
typedef pair<int, int> P;

#define rep(i, n) for(int i = 0; i < (int)(n); i++)
#define all(x) (x).begin(),(x).end()
#define rall(x) (x).rbegin(),(x).rend()

void YN(bool flg){if(flg) cout << "YES" << endl; else cout << "NO" << endl;}
void Yn(bool flg){if(flg) cout << "Yes" << endl; else cout << "No" << endl;}
void yn(bool flg){if(flg) cout << "yes" << endl; else cout << "no" << endl;}

const double PI=acos(-1.0);

int main() {
    double r;
    cin >> r;

    double ans = 2 * r * PI;

    printf("%.10f", ans);
    
    return 0;
}
