#include <bits/stdc++.h>
using namespace std;
long long get() {
  char c = getchar();
  long long x = 0LL;
  while (c < '0' || c > '9') c = getchar();
  while (c >= '0' && c <= '9') {
    x *= 10LL;
    x += (c - '0');
    c = getchar();
  }
  return x;
}
long double r;
int main() {
  ios::sync_with_stdio(0);
  cin >> r;
  r *= 2;
  r *= 3.1415926535897932384;
  cout << r;
  return 0;
}
