#include <bits/stdc++.h>
using namespace std;
long long gcdEx(long long a, long long b, long long *x, long long *y) {
  if (!a) {
    *x = 0;
    *y = 1;
    return b;
  }
  long long x1, y1, gcd = gcdEx(b % a, a, &x1, &y1);
  *x = y1 - (b / a) * x1;
  *y = x1;
  return gcd;
}
long long modI(long long b, long long m) {
  long long x, y;
  gcdEx(b, m, &x, &y);
  return (x % m + m) % m;
}
long long modP(long long x, long long y) {
  long long r = 1;
  x %= 1000000007;
  while (y > 0) {
    if (y & 1) {
      r = (r * x) % 1000000007;
    }
    y = y >> 1;
    x = (x * x) % 1000000007;
  }
  return r;
}
long long max(long long a, long long b) { return (a > b ? a : b); }
long long min(long long a, long long b) { return (a < b ? a : b); }
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  long long t;
  t = 1;
  for (long long tt = 1; tt <= t; tt++) {
    double ans = 0;
    double pi = acos(-1), r;
    cin >> r;
    ans = 2 * pi * r;
    cout << setprecision(16) << ans << "\n";
  }
  return 0;
}
