#include <bits/stdc++.h>
using namespace std;
const int inf = 2147483647, dx[] = {-1, 0, 1, 0}, dy[] = {0, -1, 0, 1};
const int N = 100005, M = 1000005, K = 200005, mod = 1000000007;
const long long llinf = 9223372036854775807ll;
double r;
int main() {
  cin >> r;
  cout << r * 3.1415926;
  return 0;
}
