#include <bits/stdc++.h>
const int M = 2e5 + 9;
const int MOD = 1e9 + 7;
const long long MOD2 = 10000004873;
const long long N = 1e12 + 7;
using namespace std;
long long pwr(long long b, int p) {
  if (p <= 0) return 1;
  long long sq = pwr(b, p >> 1);
  sq = (((sq % MOD) * (sq % MOD)) % MOD);
  if (p & 1) sq = (((sq % MOD) * (b % MOD)) % MOD);
  return sq;
}
int r;
int main() {
  scanf("%d", &r);
  cout << fixed << setprecision(20) << (double)M_PI * r * 2 << endl;
  return 0;
}
