#include <bits/stdc++.h>
using namespace std;

#pragma GCC optimize("Ofast")
#define rep(i,a,n) for(int i=a; i<n; i++)
typedef long long ll;
typedef pair<int,int> P;
const ll MOD = (int)1e9+7;
const ll INF = MOD*MOD;
const ll powll = 1LL<<1;

int main(){
	double a;cin >> a;
    cout << M_PI*a*2.00 << "\n";
}