#include<bits/stdc++.h>
using namespace std;
#define rep(i, n) for (int i = 0; i < (int)(n); ++i)
#define rep2(i, s, n) for (int i = (s); i < (int)(n); ++i)
#define fi first
#define se second
#define INF 1000000009
#define LLINF 1000000000000000009LL
using ll = long long;

int main() {
  int r;
  cin >> r;
  printf("%.10f\n", r * 2 * acos(-1));
  return(0);
}
