#include <bits/stdc++.h>

using namespace std;

#define FOR(i, n) for(int i = 0; i < (int) n; i++)
#define rall(x) x.rbegin(), x.rend()
#define all(x) x.begin(), x.end()
#define PB push_back
#define F first
#define S second

typedef long long ll;
typedef vector<int> vi;
typedef pair<int,int> pii;

const int MOD = 1e9+7;

const double PI = acos(-1);

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

	int r; cin >> r;
	cout << setprecision(8) << fixed << 2*PI*r << '\n';

 	return 0;
}