#include <bits/stdc++.h>

using namespace std;
#define rep(i,n) for (int i =0;i<(n);++i)
using ll = long long;
using P = pair<int, int>;
//int choose2(ll x){
	//	return x*(x-1)/2;}
const int INF = 1001001001;

int main() {
	ll R;
		cin >> R;
		cout << 6.28318530717958623200* R;
	
}