#include <bits/stdc++.h>
long long i, j, k, cnt, sum;
using namespace std;
bitset<10000001> prime;
void seive() {
  prime[0] = 1;
  prime[1] = 1;
  prime[2] = 0;
  for (i = 4; i < 10000001; i += 2) {
    prime[i] = 1;
  }
  for (i = 3; i * i <= 10000001; i += 2) {
    if (!prime[i]) {
      for (j = i + i; j < 10000001; j += i) {
        prime[j] = 1;
      }
    }
  }
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  int r;
  cin >> r;
  cout << r * 2 * acos(-1) << endl;
  return 0;
}
