#include<bits/stdc++.h>

using namespace std;

#define pb push_back
#define mp make_pair
#define rep(i,n)  for (int i = 0; i < n; i++)
#define loop(i,a,n)  for (int i = a; i <= n; i++)
#define deb(x) cout << #x << " = " << x <<'\n'
typedef long long ll;

int main(){
    int r;
    cin >> r;
    cout << 2*3.14159265359*r << endl;
}