#include <bits/stdc++.h>
using namespace std;
const long long M = 1e9 + 7;
long long dx[] = {1, -1, 0, 0, 1, 1, -1, -1};
long long dy[] = {0, 0, 1, -1, 1, -1, 1, -1};
long long expo(long long x, long long y) {
  long long res = 1;
  x = x % M;
  while (y > 0) {
    if (y & 1) res = (1ll * res * x) % M;
    y = y >> 1;
    x = (1ll * x * x) % M;
  }
  return res;
}
signed main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  ;
  long long t = 1;
  while (t--) {
    long long r;
    cin >> r;
    long double ans = 2 * 3.141592 * r;
    cout << ans;
  }
  return 0;
}
