#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using P = pair<ll, ll>;
const ll LINF = (1LL << 62) - 1;
const int INF = (1 << 30) - 1;
template <class T>
bool chmax(T &a, const T &b) {
  if (a < b) {
    a = b;
    return 1;
  }
  return 0;
}
template <class T>
bool chmin(T &a, const T &b) {
  if (b < a) {
    a = b;
    return 1;
  }
  return 0;
}
int main() {
  long long R;
  scanf("%lld", &R);
  cout << fixed << setprecision(10) << M_PI * R << endl;
}
