#include <bits/stdc++.h>
using namespace std;
const int MOD = 1e9 + 7;
const int INF = 0x3f3f3f3f;
const double PI = 4 * atan(1);
int r;
double dp[301][301];
void solve() {
  cin >> r;
  cout << 2.0 * PI * r << endl;
}
int main() { solve(); }
