#include <bits/stdc++.h>
using namespace std;
void fastscan(int &x) {
  bool neg = false;
  register int c;
  x = 0;
  c = getchar();
  if (c == '-') {
    neg = true;
    c = getchar();
  }
  for (; (c > 47 && c < 58); c = getchar()) {
    x = x * 10 + c - 48;
  }
  if (neg) {
    x *= -1;
  }
}
long long mypow(long long a, long long b) {
  long long res = 1;
  while (b > 0) {
    if (b & 1) res = res * a;
    a = a * a;
    b >>= 1;
  }
}
void file() {}
void CF_321_D2_A() {
  long long n;
  cin >> n;
  ;
  long long a[n];
  for (long long i = 0; i < n; i++) cin >> a[i];
  ;
  long long c = 1;
  long long ans = 0;
  for (long long i = 1; i < n; i++) {
    if (a[i] >= a[i - 1]) {
      c++;
    } else {
      ans = max(ans, c);
      c = 1;
    }
  }
  ans = max(ans, c);
  cout << ans << "\n";
  ;
}
long long a[100005];
int32_t main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  ;
  long long n;
  cin >> n;
  ;
  printf("%.2f", 2 * 3.1415 * n);
}
