#include <bits/stdc++.h>
using namespace std;
#define int long long
typedef pair<int,int> ii;
#define F first
#define S second
#define sqr(x) (x)*(x)
#define pb(x) push_back(x)
#define sz(x) (int)x.size()
#define mp(x,y) make_pair(x,y)
#define all(x) (x).begin(),(x).end()
#define rep(i,x,y) for(int i=x;i<(y);++i)
#define IOS ios_base::sync_with_stdio(0);cin.tie(0)
/* start */

const double PI = acos(-1);

signed main(){
    int r;
    cin >> r;
    cout << 2*r*PI << "\n";
}