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

typedef long long int ll;
typedef pair <ll, ll> pll;

const int Max = 2e6 + 10;
const int Mod = 1e9 + 7;
const ll Inf = 1LL << 62;

int main() {
#ifdef Mr_Emrul
        freopen("inputf.in", "r", stdin);
#endif /// Mr_Emrul

long double n;
cin >> n;
n*=2.0;
n*=acos(-1);
cout<<setprecision(10)<<n << fixed
        return 0;
}