#include <bits/stdc++.h>
using namespace std;
const int M = (int)1e2;
const int inf = 0x3f3f3f3f;
const long long mod = (long long)1e9 + 7;
const double eps = 1e-8;
int main() {
  int r;
  scanf("%d", &r);
  printf("%.12f\n", acos(-1.0) * r);
  return 0;
}
