#include <bits/stdc++.h>
using namespace std;
#define rep(i,n) for(ll i=0; i<n; i++)
#define eb(t) emplace_back(t)
typedef long long ll;
typedef long long unsigned int llu;
ll MOD = 1000000007;
ll INF = 1000000009;

ll a[1000010];

void solve(){

    long double r;
    long double PI=3.14159265358979323846;
    cin >> r;
    cout << 2*r*PI << endl;

}

int main(){
  cin.tie(0);
  ios::sync_with_stdio(false);
  solve();
  return 0;
}
